Loren
Ban
*,
Hanchao
Tang
,
Bruce L.
Yoder
and
Ruth
Signorell
*
Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog Weg 2, CH-8093 Zürich, Switzerland. E-mail: lban@ethz.ch; rsignorell@ethz.ch
First published on 4th May 2022
Photoemission from submicrometer droplets containing a mixture of dioctyl phthalate and dioctyl sebacate was investigated by femtosecond and nanosecond photoionization. Photoelectron spectra recorded after ionization with single 4.7 eV femtosecond or nanosecond laser pulses showed marked differences between the two cases. These differences were attributed to ionization of long-lived states which only occurred within the duration of the nanosecond pulse. The tentative assignment of the long-lived states to dioctyl phthalate triplet states is discussed. A nanosecond–femtosecond pump–probe scheme using 4.7 eV (pump) and 3.1 eV (probe) pulses was used to investigate the decay dynamics of these long-lived states. The dynamics showed an accelerated decay rate at higher dioctyl phthalate concentrations. Furthermore, the dependence of the decay dynamics on droplet size and charge was investigated. The decay of the long-lived states was found to be faster in smaller droplets as well as in neutral droplets compared with both positively and negatively charged droplets. Possible mechanisms to explain these observations and the dominance of contributions from the droplets surface are discussed.
Droplet PES also offers insight into fundamental properties of the condensed phase such as electron transport scattering13–18 or the electronic structure.6,19–22 In a recent publication,18 we illustrated how a charged droplet–vacuum interface influences such properties. For example, the study of low-energy electron escape from charged droplet–vacuum interfaces can provide detailed information on the shape of the interface potential. Quantum effects make low-energy electrons particularly sensitive to the height and width of the interface potential.18 Furthermore, a charged interface leads to characteristic shifts in the measured photoelectron spectra which need to be taken into account for precise determination of electronic structure in droplet PES.18
In this work, we extend our previous study of the droplet–vacuum interface and examine its influence on the photophysics near the surface. As a model system, we chose bis(2-ethylhexyl) phthalate (DEHP) dissolved in bis(2-ethylhexyl) sebacate (DEHS) droplets, where both components are liquid and their low volatility offers good control over the interface properties. The study was motivated by the observation that resonant photoionization of DEHP droplets with 4.7 eV light leads to significantly different photoelectron spectra depending on whether nanosecond or femtosecond laser pulses were used. To get at the root of this phenomenon, we performed nanosecond–femtosecond pump–probe experiments and show that following excitation at 4.7 eV, DEHP survives in long-lived states. Measurements of the electron yield decay dynamics showed these long-lived states to decay on a microsecond timescale. The dynamics turned out to depend on the DEHP concentration as well as on the droplet’s size (500 nm and 1200 nm in diameter) and charge state (neutral and charge states of up to about ±20e). These results suggest that surface effects could play a role in the decay of the long-lived states. Although still at an early stage, this work illustrates the prospects of droplet PES for time-resolved studies of photoinduced processes in submicrometer systems where surface effects can lead to enhanced reactivity.
Typical droplet size and charge distributions obtained from the SMPS are shown in Fig. 2 and 3. The size-selected (Da = 500 nm) sample shows a single peak with a full-width at half-maximum of about 90 nm. The corresponding charge distributions for positively and negatively charged samples are shown in Fig. 3. The average droplet charge states of +15e, +20e and −17e are obtained, with a standard deviation of a few charge units.
The photoelectron spectrometer is operated in a Velocity Map Imaging (VMI) configuration, with two different detection modes employed in this work. In the first part of the paper, we use the camera to record angle-resolved photoelectron images. Photoelectron kinetic energy (eKE) spectra are retrieved by image reconstruction along the laser propagation axis1 with MEVIR26 and background-corrected by subtraction of an image collected in the absence of the droplet beam (with laser beams on). In the second part of the paper, we decoupled the electron signal directly from the imaging detector and recorded electron time-of-flight (eTOF) spectra to retrieve total photoelectron yields. Recording photoelectron yields requires the collection of fewer laser shots to achieve a reasonable signal-to-noise ratio and was therefore used to perform pump–probe studies over a wide range of time delays (see below).
The images are asymmetric along the laser propagation axis, with signal found on the image half away from the incident laser direction (single-headed arrow in Fig. 5). This is a characteristic feature of VMI PES of weakly-absorbing droplets that show so-called nanofocusing.1,5,28 A clear difference between the ns and fs images can be seen, with the signal in the nanosecond case extending towards larger radii, i.e., higher electron kinetic energies.
For a more quantitative comparison, the corresponding photoelectron spectra are shown in Fig. 5c. The spectra are normalized to their respective signal at eKE ∼ 0.2 eV. The nanosecond spectrum (red) shows two bands that extend up to electron kinetic energies (eKE) of ∼3 eV. The spectrum is in good agreement with our previous study on pure DEHP droplets.18 By contrast, the femtosecond spectrum (black) shows about one order of magnitude lower signal at eKE > 1 eV (relative to the signal at 0.2 eV) and the signal seems to extend to higher eKE (see difference at ∼3 eV).
Fig. 6 Droplet photoelectron spectra (circles) from Fig. 5 shown on a logarithmic y-scale. The spectra are fitted by a multi-component Gaussian function (full-lines, components shown as dashed, dashed-dotted and dotted lines). Panel (a) shows nanosecond and panel (b) femtosecond ionization. Individual onset values are indicated by vertical gray dotted lines. |
Using eKE onsets (to be compared with the AIE) instead of peak maxima (to be compared with the VIE) circumvents some of these issues. The eKE onsets determined from the multi-component fits are qualitatively compared with the calculated ionization energies (IE, Table SI1).†
The resulting fits to the data are shown as full lines in Fig. 6, with dashed lines representing individual components. The nanosecond case (Fig. 6a) is well fitted by 2 component bands with onsets of eKEσ(ns,I) = 1.9 eV and eKEσ(ns,II) = 3.0 eV and similar peak amplitudes. In the femtosecond case (Fig. 6b), 3 components were necessary to fit the data with eKEσ(fs,I) = 1.3 eV, eKEσ(fs,II) = 4.2 eV and eKEσ(fs,III) = 6.0 eV. The first component (fs,I) has about one and two orders of magnitude higher amplitude than components two (fs,II) and three (fs,III), respectively.
First, we consider the 1 + 1 REMPI process indicated by (1) in Fig. 7. Absorption of the first 4.7 eV photon leads to resonant excitation (blue arrow). Absorption of an additional photon is enough to reach the ionic state manifold (D0, Dsolv0) completing the 1 + 1 REMPI (1). The calculated condensed phase AIE (see ESI, Table SI1)† yields an eKE onset value of 1.6 eV in reasonable agreement with the experimentally determined eKEσ(ns,I) of 1.9 eV and eKEσ(fs,I) of 1.3 eV. Therefore, we assign these bands to a 1 + 1 REMPI from the ground state. The difference in the onsets of ∼0.6 eV might be ascribed to the overlap of the (ns,I) band with the asymmetric low eKE tail of inelastically scattered electrons of the strong (ns,II) band. The fit accounts for this by increasing the width of the lower eKE Gaussian component, thereby shifting eKEσ(ns,I) to correspondingly larger values. This shift is virtually absent in the fs spectrum because the (fs,II) band is much weaker than the (fs,I) band.
On the premise that ionization requires a minimum of 2 photons (process (1)), the difference between the ns and fs spectra could arise from the presence of higher-order (1 + n) REMPI. These processes are much more probable in the fs case due to the shorter pulse duration and, in turn, significantly higher peak intensities (GW cm−2 in the fs case, kW cm−2 in the ns case). Since bands originating from (1 + n) REMPI would be shifted to higher eKE by integer multiples of the photon energy, we can assign the eKEσ(fs,III) band at 6.0 eV to 1 + 2 REMPI (eKEσ(fs,III) − eKEσ(fs,I) = 4.7 eV). Significantly weaker amplitude of the 1 + 2 REMPI band is reasonable since it includes an additional non-resonant step which has a significantly lower cross section.
However, bands eKEσ(ns,II) at 3 eV and eKEσ(fs,II) at 4.2 eV cannot be explained in this way. Their eKE is too high to be explained by the 1 + 1 REMPI and too low to be explained by the 1 + 2 REMPI. This indicates that the excited-state dynamics in the Sn manifold taking place within the pulse duration plays a role.29,30 The effect of this dynamics will be very different in the ns case and the fs case. Only ultrafast (<70 fs) relaxation processes are expected to play a role in the fs case, while slower processes such as intersystem crossing can occur within the duration of the ns pulse. In both cases, all the states generated during these dynamics can absorb additional photons and be ionized. This picture is illustrated by the “ladder” and “ladder switching” models.35 We propose that the bands eKEσ(ns,II) and eKEσ(fs,II) originate from distinct molecular states formed as a result of the dynamics initiated by single-photon excitation. These intermediate states are ionized by additional photons within the duration of the same ns or fs pulse.
The (fs,II) band can be assigned to 2-photon ionization of the S1 state upon fast internal conversion of the Sn manifold, designated as process (2) in Fig. 7. The calculated values of the condensed phase AIE and the S1 energy lead to an estimate of the eKE onset value of 4.8 eV, in reasonable agreement with the experimental onset eKEσ(fs,II) = 4.2 eV. Part of the difference might be ascribed to geometry relaxation in the S1 state not accounted for in our DFT calculations. Our assignment implies an internal conversion of the Sn manifold to S1 state on a sub-100 fs timescale, which is not unlikely considering literature on similar systems.36–38
The long duration of the ns pulse (∼7 ns), by contrast, also allows for slow relaxation processes to take place (e.g. intersystem crossing, photofragmentation and solvated electron formation). For the ns band we determined an onset eKEσ(ns,II) = 3.0 eV which would lead to IE = 6.4 eV (2-photon ionization) or IE = 1.7 eV (1-photon ionization). The IE of 1.7 eV is rather low and could only be explained by weakly bound molecular states such as anions formed by electron attachment after 2-photon ionization by process (1). It might also be conceivable, that weakly-bound solvated electrons are formed by localization of a photoelectron generated in process (1) – analogous to the above-mentioned electron attachment. The low-polarity of the solvent DEHS (ε ∼ 4),34 however, makes these options appear unlikely. In addition, solvated electrons were not reported in previous studies on similar systems.39 One might also think of anionic species formed along the fragmentation pathway of DEHP. However, DEHP fragments formed by UV dissociation which are reported in the literature include only neutral and cationic species which are unlikely to have such a low IE.40–42
Discarding major contributions from such 2 + 1 processes, i.e., process (1) followed by single photon ionization, we are left with the possibility of a 1 + 2 process. We propose to assign the (ns,II) band to the 2-photon ionization of long-lived states formed after photoexcitation to the Sn manifold, as indicated by process (3) in Fig. 7. The strong contribution of the (ns,II) band to the spectrum indicates that the efficiency of this ionization channel is comparable to the direct (1 + 1) REMPI channel (process (1)). This implies that the long-lived states are formed with a high probability and that it has a comparable ionization cross section to the ground state.
In the ns case, the intensity of the (ns,II) band relative to that of the (ns,I) band increases with decreasing concentration. By contrast, the fs spectra show similar relative band intensities for all concentrations. This illustrates that the (fs,II) and (fs,III) bands are only minor spectral components whose relative contributions do not depend on the concentration of DEHP. This is consistent with the proposed origin of these bands in fast unimolecular relaxation (internal conversion) and higher order ionization.
The pronounced concentration dependence in the ns case (Fig. 8) suggests that the contribution of the (ns,II) band is sensitive to the surrounding DEHP molecules. The relative contribution of the (ns,II) band decreases at higher DEHP concentrations. This behavior suggests that the long-lived states are efficiently quenched at higher concentrations by additional DEHP molecules. While this simple mechanism qualitatively explains the data, we note that a complex interplay with additional mechanisms (e.g. bimolecular association to form a DEHP dimer) could also be consistent with the experimentally observed concentration dependence (see below).
Fig. 10 compares the resulting pump–probe spectrum (black) with the single-pulse nanosecond case (red). The pump–probe spectrum (black) contains a clear contribution from a high-energy band that agrees well with the (ns,II) band of the single-pulse nanosecond experiment (red). This supports the assignment presented above, indicating that the long-lived states are generated during the ns pump pulse (7 ns) and persist for at least 30 ns before they are ionized by the fs-probe. The VIE and the eKE onset are in good agreement with the single-pulse nanosecond spectrum (red). While we cannot exclude that the long-lived states undergo further transformations after the ns-pump pulse, these transformations do not seem to affect their photoelectron spectrum significantly. Therefore, we conclude that the differences in photoionization by femtosecond and by nanosecond pulses (i.e. the (ns,II) band) are due to strong contributions from long-lived states formed during the nanosecond pulse.
A weak feature in the pump–probe spectrum at eKE ∼ 2.8 eV might be attributed to an artefact of the image reconstruction or the background subtraction. At low kinetic energies (eKE < 0.7 eV) the strong single-pulse femtosecond contribution (Fig. 5) in the background image causes significant scatter in the pump–probe signal after background subtraction so that meaningful analysis of this eKE range is currently impossible.
Fig. 11 shows the decay dynamics of the electron yield Y(Δt) for t > 10 ns (just outside of the instrument response function) as a function of the DEHP concentration. All data are normalized to the value Y(Δt = 10 ns) of the measurement at the highest concentration (1.5 M). The experimental data (circles, shaded areas are the estimated signal uncertainties) show that the electron yield systematically increases with increasing DEHP concentration. As such this is no surprise as the number of absorbing DEHP molecules in the probe volume increases. However, this dependence is complicated by additional fast (Δt < 10 ns) decay channels that become important at higher DEHP concentrations. The decay dynamics shows a concentration dependence, with faster signal decays at higher DEHP concentrations, evidenced in Fig. 11 by the decreasing spread of the yield traces on the logarithmic scale. Thus, the ratio of the yields at the latest and earliest delay times increases from at a concentration of 1.5 M and to a value of 0.23 at a concentration of 0.0025 M. While the long-lived states invariably survive in significant amounts for at least 5 μs, their decay accelerates in more concentrated DEHP solutions.
A commonly encountered long-lived state in the literature on similar aromatic systems is the triplet (T1) state. For example, efficient and fast formation of the triplet T1 state following excitation to the Sn manifold was reported for the benzoic acid monomer in the gas phase43–45 and in acetonitrile.38 In the latter, photoexcitation at 267 nm resulted in the formation of T1 (∼2.5 ps) with a large quantum yield (∼0.65). No photofragments were observed. A T1 lifetime of 9 μs was measured in N2 saturated solutions and 0.1 μs in air saturated solutions. The photo-physics of DEHP was reported in ref. 46 following excitation with 20 ns, 266 nm pulses in dilute 1,1,2-trichlorotrifluoroethane solutions. The authors report that long-lived triplet states decay via a first-order decay process (∼80 ns) in O2 saturated solutions, while second-order triplet–triplet annihilation kinetics was reported in Ar-saturated solutions with half-lives of a few microseconds, leading to deactivation into the singlet manifold and to the formation of biradicals on a longer timescale (∼20 μs). Similar results were found in smaller analogues of DEHP, such as dibutyl phthalate,39 where triplet states were also suggested as primary photoproducts.
From the above, it seems reasonable to assume that a likely candidate for the long-lived states is the triplet (T1) state. This assignment is supported by the calculated IE (5.4 eV for VIE and 4.6 eV for AIE, see ESI, Section 2)† which is in reasonable agreement with the experiment (Fig. 5 and 10). Contributions from other candidates, such as photo-fragments or solvated electrons do not appear plausible for the same reasons already given for the band assignment in the ns single pulse PES (see above).
Let us now consider the decay dynamics (Fig. 11) as a further test of the triplet state assignment. The photophysics of the triplet (T1) state47 includes various unimolecular processes (e.g. phosphorescence and intersystem crossing) and efficient quenching reactions by contaminants (most often O2). The intrinsic T1 lifetimes (in the absence of quenchers) are typically on the order of microseconds, so that the <5 μs decay (Fig. 11) must have another explanation. Provided quenchers are present in excess (or if the process is unimolecular), the quenching follows pseudo-first order kinetics, described by a single exponential decay with a rate constant equal to the sum of rate constants for all quenching reactions starting from the same initial state.
Therefore, in a first phenomenological approach we use an exponential decay model to analyze the data. A minimum of three independent exponential decay terms were required to obtain a reasonable fit to the data in Fig. 11
Yfit(t) = a1exp(−k1t) + a2exp(−k2t) + a3exp(−k3t) |
The need for three exponential decays suggests that the electron yield dynamics cannot be explained by triplet quenching processes only (e.g., oxygen quenching). Either additional decay channels (e.g., bimolecular processes) or additional states seem to contribute to the signal (e.g., intermediate states or initial states other than the triplet). It is currently not possible to assign these three timescales to a physical reaction mechanism. Further experiments examining the effect of quenchers (i.e., oxygen) on the decay kinetics are necessary since in our present experiment the oxygen content could not be determined or controlled. As a caveat, we note that a sufficient number of exponential decays can eventually describe any decay dynamics, even when physical mechanisms other than first-order reactions are at play.48
As an alternative picture let us consider a situation where the above-mentioned quenching reactions are absent and only bimolecular processes (self-quenching and triplet–triplet annihilation46,47) are observed on the time scale of the experiment. We found that the following ansatz leads to a reasonable fit to the data in Fig. 11
Finally, we would like to note that our electron yield measurements cannot directly distinguish triplet state contributions from other intermediate states with a similar IE. In both approaches (exponential and diffusion-limited) the involvement of intermediate states would entail more complicated kinetic expressions to reproduce the data with a physically meaningful parameterization. For example, excimer (excited DEHP dimer) formation is one of the outcomes of the triplet–triplet annihilation process.47 Since their IE is similar to the triplet state, the electron yield could well contain contributions from both species.
In conclusion, we cannot at this moment assign the electron yield decay dynamics to a precise physical mechanism. We analyzed the data considering two limiting cases, composite first order (exponential) kinetics and diffusion-limited kinetics. Neither model affords a conclusive analysis of the data in terms of physically interpretable parameters, possibly indicating that intermediate levels are involved in the observed “long-lived states” dynamics. Further kinetics experiments are necessary to obtain a clear picture, e.g., by quantifying the influence of experimental parameters such as the oxygen concentration, the viscosity of the liquid or the temperature.
The electron yield shows a decay rate which decreases with increasing droplet diameter from 500 nm (black) to 1200 nm (red) to a few micrometers (green).
How can the observed size-dependence be explained? First, we need to consider the effects of inelastic electron scattering which determines the experimental probing depth by limiting the distance from the surface from which photoelectrons can escape to vacuum and be detected. For water, the probing depth of <10 nm at an eKE of a few eV results from a value of ∼3 nm for the inelastic mean-free path (IMFP).14,31 Taking data on solid benzene49,50 as a rough estimate for the IMFP in DEHP/DEHS mixtures we arrive at a range of ∼1–10 nm. The probing depth is likely slightly higher than in water (a lower density of scatterers leads to a longer IMFP), but presumably does not exceed ∼10 nm. Consequently, we can assume that our measurements (Fig. 12) are only sensitive to long-lived states that are present within a thin layer less than ∼10 nm away from the surface. The range of diffusion in DEHP/DEHS droplets on a timescale of ∼200 ns to ∼2 μs lies between ∼2 nm and ∼6 nm ( assuming D = 10−2 nm2 ns−1).51 These distances are smaller than the probing depth and indicate that the long-lived states do not diffuse out of the layer defined by the probing depth within our pump–probe time-window.
In addition to the probing depth, the light intensity distribution inside the droplet must be taken into account. For droplets with diameters in the submicrometer range, interacting with visible or UV light, the internal light intensity distribution is not homogeneous,1,5,13 leading to spatially dependent photoexcitation and photoionization probabilities. Therefore, it is important to examine whether the observed size-dependence could arise from the internal electric field magnitude or size-dependent spatial overlap of pump and probe pulses. The probability of forming long-lived states (we assume a triplet) in an infinitesimal volume dV is proportional to
[T1(dV)] ∝ ΦTσS[S0]Ipump(dV) |
Y(dV) ∝ σT[T1(dV)]Iprobe2(dV) |
Y(dV) ∝ Ipump(dV)Iprobe2(dV) |
We used the discrete-dipole approximation52 to calculate Ipump(dV) and Iprobe(dV)2 assuming refractive indices of N = 1.52 + i0.001 (4.7 eV) and N = 1.49 + i0.0001 (3.1 eV). For the real part of the refractive index we used values for DEHP13 (DEHS is assumed to have the same value53). For the imaginary part, we scaled the DEHP values at the corresponding wavelength by 0.1 to account for a concentration of 0.2 M (DEHS does not significantly absorb at these wavelengths).
Fig. 13 shows the resulting internal light intensity distributions in the plane spanned by the laser propagation and polarization directions. The color scale shows the logarithm of the internal light intensity relative to the incident light intensity. The effect of nano-focusing is clearly visible in the asymmetry of the distribution and the intensity enhancement which increases for larger particles (left to right). Iprobe(dV)2 (top row) shows a large light intensity enhancement with particle size, with maximum values increasing by almost 1 order of magnitude going from a particle diameter of 500 nm to 1000 nm (see color bars). By comparison, Ipump(dV) increases by a factor of ∼2 in the same size range. The larger enhancement in the probe process (Iprobe(dV)2) reflects the dependence of the 2-photon ionization probability on the square of the intensity.
The most obvious effect of droplet size is the different magnitude of light intensity enhancement. Data in Fig. 12 are normalized to signal at ∼10 ns so that total relative signal at longer times should reflect the survival probability of the long-lived states since
In addition, the spatial distribution of the light intensity needs to be considered. From Fig. 13, the spatial distribution of both pump and probe pulses seems qualitatively similar. To investigate the spatial distribution of the light intensity in more detail, we integrated the 3-dimensional internal light distributions over the whole solid angle (Ω)
I(r) = ∫∫I(r,Ω)r2dΩ |
The internal light intensity distribution of pump pulses Ipump (Fig. 14a) is maximal close to the surface and extends further into the droplet interior for larger droplets. In the probe case, Iprobe2 (Fig. 14b), the distribution shows a pronounced maximum close to the surface and a weaker tail extending to the droplet interior for larger sizes. The product IpumpIprobe2, which determines the spatial overlap of our pump–probe experiment is shown in Fig. 14c. It shows one dominant feature close to the droplet surface. For analyzed droplets diameters this feature is essentially size-independent up to ∼30 nm away from the surface. This distance is larger than the probing depth (<10 nm) and we can therefore conclude that the size-dependent distribution of light inside the droplets plays no role on the observed decay dynamics.
From the above, it is clear that the observed increase in the electron survival probability with droplet size cannot be explained by diffusion out of the finite probing depth nor by variations in the internal light distribution. This leaves the influence of the droplet surface on the reactions taking place within a thin layer <10 nm from the surface as a possible cause for the observed size-dependence of the dynamics.
Accelerated reaction rates have been previously reported in micrometer sized droplets, but their physical origin is still being discussed.54 A possible explanation can be proposed in the case of diffusion-limited reaction, where the reaction time is lowered by the decreased dimensionality at the droplet surface.55 This constraint is relaxed as the droplet size increases and the reaction slows down, in qualitative agreement with our data. This hypothesis needs further experimental testing and must be interpreted with care. As highlighted in ref. 54, detailed and critical analysis of different effects is necessary before assigning the altered kinetics to a particular surface effect.
In Fig. 15, we show the electron yield decay dynamics recorded on the droplets with a diameter of 500 nm with varying degrees of charging. The electron yield is normalized to its signal at ∼10 ns. The charge states correspond to the measurements shown in Fig. 3. The droplet charge is assumed to be homogeneously distributed on the droplet surface. This assumption is supported by computational studies on electrospray ionization.54,57,58 In this picture, the electric field magnitude E on the droplet surface of diameter d can be calculated from the droplet charge state (q = ne) by the simple model of a charged conductive sphere (see also ref. 54)
The number density of the charged droplet sample was significantly lower than in the neutral case, which is reflected in the larger signal uncertainty. The temporal evolution of the electron yield of charged droplets clearly deviates from the neutral sample and leads to higher electron yield at times above ∼200 ns. The electron yield is enhanced for both charge polarities and no clear dependence on the electric field magnitude was observed within the relatively large uncertainty.
First, it is important to discuss the nature of the droplet charging process since we are investigating excited state photophysics where quenching by impurities could have a strong effect. The droplets are charged by collisions with ions generated by the discharge in the gas flow. The species responsible for charging therefore depends on the polarity of the discharge, specific charge settings as well as the gas composition. In air, positive corona discharge primarily consists of small, protonated water clusters.59 The situation is more complicated in the negative case,60 where the generation of ozone leads to the presence of hydrated cluster anions of OH−and NO3−. The number of absorbed ions directly determines the droplet charge, which at the charge states used in our experiment corresponds to a negligible concentration of these ions. The effects of ions on the photo-physics can therefore be safely neglected and cannot explain the difference from the neutral case. This is further confirmed by the fact that both polarities (different ions) show the same effect even though different ions would be formed.
But how then can the presence of charge influence measured photo-electron yields? Let us consider two general mechanisms by which an electric field can affect the electron yield. The first one applies to ionization processes that proceed via charge-pair states. In this case, the electric field can enhance the electron signal by facilitating charge separation and thus increasing the ionization probability. Examples of this mechanism are found in benzene excimers excited to charge-transfer states just below the ionization threshold.33 Electric fields of about 10−3 V nm−1 produced a measurable increase in free charge generation.
The second mechanism works through the influence of the electric field on the diffusion of a particle. This has been widely investigated in the context of the recombination of charged particle pairs61 (e.g. solvated electron) and of neutral-charged pairs.62 For ion-pair geminate recombination,61 electric fields of ∼10−2 V nm−1 resulted in an increase in the escape probability. However, in the case of neutral reactants, the effects of the electric field are not obvious.
The electric field effect on the electron yield could bring additional insight into the decay mechanism. Within the assumptions of our kinetic model, we can exclude the effect of charged particle diffusion since only neutral species are considered. We therefore propose that surface charge facilitates the charge-pair separation that takes place during the ionization process.
The electric charge effects could be a hint that excimer states, in addition to the triplet state, could contribute to the signal since their ionization often proceeds via a charge-separation mechanism.33 This could be rationalized by a triplet–triplet annihilation mechanism which proceeds via an excimer intermediate (T + T ⇆ E →). Future studies should therefore extend the kinetic analysis to account for the possible effect of diffusion-limited monomer-excimer reactivity.63,64 An intriguing observation is that our exponential kinetic model yields a time constant of ∼240 ns, commensurate with the time delay after which the charged droplet electron yield departs from the neutral case in Fig. 15.
We further investigated the influence of particle size and charge state on the decay dynamics of the electron yield. We observed a decrease of the decay rate with increasing droplet size. Because of the relatively low probing depth (<10 nm), the experiment preferentially probes the kinetics near the surface. Therefore, we tentatively attribute the decrease of the rate observed for larger droplets to an altered diffusion of the reactants near the surface. Further experiments are necessary before a detailed mechanism can be proposed. The decay rate of the electron yield also decreases in charged compared with neutral droplets. This effect does not depend on the charge polarity. We propose that the electric field on the surface of the droplets increases the ionization probability by facilitating charge-separation in the ionization process. This result hints that excimer states could be important intermediates in the long-lived states decay kinetics.
Although many questions remain open, the present study highlights the potential of droplet photoelectron imaging to gain insight into the photophysics and photochemistry in confined systems. The combination of pump–probe photoelectron spectroscopy with droplet beams of controlled size (submicrometer regime) and charge enables the study of kinetics at a surface with high time resolution.
Footnote |
† Electronic supplementary information (ESI) available: Additional information on the experimental and theoretical methods. See DOI: 10.1039/d1fd00108f |
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