Inner-shell X-ray absorption spectra of the cationic series NH y + ( y = 0–3)

Ion yields following X-ray absorption of the cationic series NH y + ( y = 0–3) were measured to identify the characteristic absorption resonances in the energy range of the atomic nitrogen K-edge. Significant changes in the position of the absorption resonances were observed depending on the number of hydrogen atoms bound to the central nitrogen atom. Configuration interaction (CI) calculations were performed to obtain line assignments in the frame of molecular group theory. To validate the calculations, our assignment for the atomic cation N + , measured as a reference, was compared with published theoretical and experimental data.


Introduction
To understand the function of large biomolecules, like amino acids and peptides, one has to study the properties of their fundamental constituents, i.e. their electronic and geometric structure, 1-4 as well as their dynamics. [5][6][7] By exploiting the element specificity of inner-shell excitation, X-ray absorption spectroscopy (XAS) is an ideal tool to study the local electronic and geometric structure in the vicinity of a specific element within a molecule. Because of the rather large number of atoms within a biomolecule, photoabsoprtion spectra can be complex and sophisticated calculations are needed for their interpretation. 8 Thus, modelling accurate X-ray absorption spectra for biomolecules is very challenging. 9,10 Instead of addressing an entire biomolecule, one can interpret the (measured) X-ray absorption spectrum as being composed of the contributions of smaller constituents or building blocks of larger molecules. For example, glycine (NH 2 -CH 2 -COOH), the simplest amino acid, connects the amino group NH 2 and the carboxyl group COOH via a methylene bridge. Solved in water, like all amino acids, glycine is a zwitterion composed of a protonated amino group R-NH 2 + and a de-protonated carboxylic acid COO À at its iso-electric point. Based on the assumption that the X-ray absorption is dominantly determined by the local chemical environment, knowing the XAS of NH 2 + and COO À will facilitate the interpretation of more complicated XAS of e.g. glycine.
An inherent difficulty of this approach is that both of these building blocks of glycine are transient ionic molecules which are (almost) impossible to prepare as a solid or as a solution in water. However, an ion beam arrangement can be used to deliver ionic samples to the X-ray interaction region to address their absorption spectrum. [11][12][13][14][15] Mass spectroscopy with high resolving power does not only offer the possibility to prepare a pure, transient molecular ion beam, but additionally provides control about the chemical environment of the molecular ion, enabling or disabling the interaction of chosen elements within the molecule (see Fig. 1). Depending on the number y of hydrogen atoms bound to the central nitrogen atom in NH y + , the electronic structure of the respective molecular ion and hence its absorption features will change. Especially inner-shell resonances, like the nitrogen 1s-2p resonances, are sensitive to the chemical environment of a molecule (see e.g. ref. [16][17][18], enabling us to probe the changes in the electronic structure of the respective molecular ions by performing a XAS-experiment. Whereas there is plenty of previous work on the neutral species of the series NH y , [19][20][21][22][23][24][25][26][27][28][29][30] little work on the cations NH y + ( y = 1-3) has been published. Theoretical work on the electronic structure of NH + 31,32 and NH 2 + 33 has been done calculating molecular potential curves and bonding angles. Experiments on molecular ions addressed the valence electronic structure of NH 2 + using VUV photoionisation 34 or the determination of the a Deutsches Elektronen-Synchrotron (DESY), Notkestr. 85, 22607 Hamburg, absolute cross section of NH + using electron impact ionisation. 35 Yet, there is no detailed information available on the inner-shell XAS of the cationic series NH y + apart from the atomic cation, N + , which has been studied previously by Gharaibeh et al. 36 Furthermore, the role of NH y with different numbers y of hydrogen atoms has recently been studied in the liquid phase by Ekimova et al. for alkylamines in different solutions. 37,38 They show a strong dependence of the N 1s X-ray spectra on the number y of H atoms and the solution itself.
Here, we report on our systematic investigations on the X-ray absorption for the cationic series NH y + (y = 0-3) in a combined experimental and theoretical work.

Experiment
The PIPE merged-beams set-up 39,40 at the XUV beamline P04 41 of PETRA III, DESY, was used to record XAS spectra of NH y + in the photon energy range of the nitrogen K-edge. Details of the preparation of a molecular ion beam at PIPE have been given in a previous publication. 42 As primary source for the production of the target molecular cations we have used gaseous NH 3 . The gas leaked into the plasma chamber of a 10 GHz Electron Cyclotron Resonance (ECR) 43 ion source via an electronic needle valve keeping the gas pressure in the order of 2 Â 10 À5 hPa. Fig. 1 shows the mass spectrum of the molecular cation series of NH y + (y = 0-3). As can be seen, the resolving power of the preparation dipole-magnet mass spectrometer is sufficient to distinguish each single ion of the series. Both elements, nitrogen and hydrogen, have two naturally stable isotopes but the abundance ratio for both cases, 14 N/ 15 N and 1 H/ 2 H, is in the order of 0.99/0.01. 44 Thus, the composition of the mass lines in Fig. 1 can be considered to be almost pure. A possible NH 4 + molecular ion has the same mass-to-charge ratio as H 2 O + . However, the corresponding absorption spectrum did not show any nitrogen resonances, indicating that no NH 4 + molecular ions were produced in our ion source.
A beam of singly charged molecular ions was prepared and accelerated to 6 keV kinetic energy, transported to the interaction region on ground potential and collinearly overlapped with the X-ray photons of P04 along an effective absorption length of 1.7 m. The vacuum chamber base pressure of the interaction region was in the order of 1 Â 10 À9 hPa.
At the nitrogen K-edge, the X-ray photons create an innershell hole in the ionic target which results in subsequent Auger decay. The excitation and ionisation is accompanied by possible shake-off processes and is followed by dissociation producing N q+ (q = 2, 3) ions. These ions were separated from the target ionic beam with a second magnetic-dipole mass spectrometer. The N q+ (q = 2, 3) ions have been counted with a channeltron as a function of the photon energy.
The yield of photoexcited ions of an atomic nitrogen ion N + target was measured as a reference to help identifying the spectroscopic features for the molecular targets, as recently done for IH + . 42 The X-ray photons delivered by the P04 beamline of PETRA III were dispersed by a 400 lines per mm variable line spacing grating (VLS) favouring high photon flux in the order of 1 Â 10 12 photons per s to 7.5 Â 10 13 photons per s for monochromator exit slits from 20 mm to 1500 mm. The molecular ion yields were measured with a monochromator exit slit of 500 mm and for the atomic ion yield a slit width of 50 mm was chosen. This results in an energy bandwidth in the order of DE = 0.5 eV for the molecular spectra and DE = 0.05 eV for the atomic spectrum. The energy axis of the P04 monochromator was calibrated with the known resonance positions of the atomic N + 1s -2p resonance group 36 (see Fig. 3a). The resulting uncertainty of the experimental photon-energy scale is estimated to be approximately 0.1 eV.

Theory details
To assign the observed ion yield peaks with X-ray absorption resonances, we have conducted X-ray absorption cross section calculations with the XMOLECULE toolkit. 45,46 The calculations are based on geometries obtained from geometry optimisation for the respective NH y + cationic ground state using GAMESS 47 on the ROHF/MP2 level employing a 6-311G(d,p) basis set. 48 We have calculated the orientation-averaged X-ray absorption cross section, where in eqn (1) o = E f À E i and |c i i, |c f i are the initial and final electronic states with energies E i and E f , respectively. The quantity hc i |r|c f i is the transition dipole matrix element between the initial and final electronic state, a C 1/137 is the fine structure constant, and h = 1. For NH 3 + and NH 2 + , the initial cationic states, |c i i, were described with orbitals obtained from a single-configuration-restricted open-shell Hartree-Fock procedure. The initial states |c i i for NH + and N + have been described based on a complete active space self-consistent field calculation with an active space constructed by the 3s and the two showing clearly separated mass lines.
p orbitals for NH + and the three atomic 2p orbitals for N + , respectively. These calculations are part of a new implementation in the XMOLECULE toolkit. The final states |c f i in eqn (1) were described in a configuration interaction (CI) calculation employing the same orbitals as for the cationic initial state. The CI calculations included all configurations that are constructed from the cationic reference configurations by promoting a single electron from the K shell to an unoccupied orbital. Moreover, further configurations were included involving an additional excitations of the valence electrons into virtual/singly occupied orbitals. The calculations were conducted using spin-and symmetry-adapted configuration state functions employing the respective discrete point symmetry of the molecule. In cases where the point symmetry of the molecule is not discrete (e.g. D 3h ) the irreducible representation of respective states was identified via post-processing of the calculated results. For all the X-ray absorption calculation we used a 6-311G basis set augmented with additional 6 diffuse basis functions of angular momentum s, p, and d, respectively, according to the recipe of Kaufmann et al. 49 For efficient integral evaluation XMOLECULE employs the libcint library. 50 Although the employed basis set contains many diffuse functions, it becomes insufficient for very high valence excitations approaching the electronic continuum. Therefore, the agreement with experimental spectra is expected to decrease with higher X-ray excitation energies. Apart from this discrepancy there is a certain imbalance in the description of initial and final electronic states. This imbalance leads to a calculated transition energy which is slightly too low. Thus, we obtain calculated X-ray absorption lines that have lower energies by 1 eV to 2 eV compared to the experimental data. For better comparison between experimental and theoretical results, we have shifted the calculated absorption lines to higher values, as specified in detail within the results section. In particular, we find that for all molecules the photonenergy range E ph 4 405 eV involves a slightly smaller energy shift compared to E ph o 405 eV. Furthermore, the applied shift slightly varies along the series NH y + (y = 0-3). We attribute this varying relative shift to slight differenences in the orbital relaxation of the different molecules and respective excitation ranges. A similar approach for calculating X-ray absorption spectra has been employed in earlier works 51,52 but with a slightly different configurational space. The mentioned energy shift relative to the experimental data could potentially be avoided by using an orbital set for the final core excited state that has been optimised in the presence of the core hole and might result in an improved modelling of core-hole relaxation effects. However, the calculation of electronic transitions (eqn (1)) becomes computationally very demanding with different orbital sets for initial and final state in combination with a large configuration space. We think that the resulting energy shift of 1 eV to 2 eV is acceptable for the purposes of our study and therefore decided keep a common orbital set.

Results and discussion
The normalised ion yields Y measured for the target ion series NH y + ( y = 0-3) are shown in Fig. 2 from y = 0 (panel a) to y = 3 (panel d).
The curves consist of the N 3+ ion yield for the atomic N + target and the N 2+ ion yield for the molecular targets NH y and N + product channels, for the N + and NH + primary beams, respectively, exhibited excessive backgrounds from collisions with residual gas targets such that no meaningful photoinduced ion yields could be measured in these product channels. The normalised photoinduced ion yield Y is given by the background-subtracted ion count rate R measured with the channeltron detector divided by the target ion current I ion and photon current f ph , In principle, the PIPE setup allows one to put the measured ion yields on an absolute cross-section scale. 39 However, for the View Article Online present experiment, there are two unknown quantities: the transmission of the molecular fragments from their point of origin to the detector as well as the exact branching ratio into the measured fragment channel. Still, we can assume that the normalised ion yield Y is proportional to the partial absorption cross section of the respective fragment channel N q+ . Upon absorption of an X-ray photon in the energy range of the nitrogen 1s orbital, the ionic targets can either be resonantly excited or, above the threshold, photoionised. In both cases, an inner-shell hole remains which can subsequently decay by an Auger process, thereby changing the charge state of the target.
To obtain the observed final fragment charge state q = 3 after resonant excitation of N + two electrons need to be emitted from the resulting 1s2s 2 2p 2 np configuration. For configurations where a two-step Auger cascade is energetically not allowed, i.e. for n = 2, in particular, a double Auger process is required. Such processes are known to appear with a probability of a few percent following atomic K shell ionisation of light elements. 40,[53][54][55][56][57][58] For the molecular ions NH y are more pathways, resonant and non-resonant, to create the detected N 2+ fragment compared to the two ionisation processes needed for the N + target to reach N 3+ , since hydrogen partners can leave the system either as neutral atoms or as protons. N 2+ , for instance, can be created from NH + via single Auger decay and release of a neutral hydrogen or via double Auger decay and release of a proton. From NH 2 + and NH 3 + the creation of N 2+ ions can be expected to be less likely, since it requires more hydrogen to leave the molecule as neutral atoms. This trend is reflected in our data by a steep drop of the N 2+count rate when gradually changing the molecular target from NH + to NH 3 + (with each additional hydrogen the ion yield drops by an order of magnitude). However, since our experimental set-up does not allow us to detect neutral fragments or N + ions, we have no measure on competing fragmentation channels. Distinct characteristic behaviour can be observed in the ion yield spectra of Fig. 2. On the one hand, below the 1s ionisation threshold a strong resonance group is observed in each spectrum which originates in the atomic case from the transition 1s -2p. At higher energies, except for NH + , the first 1s ionisation threshold is visible together with Rydberg resonances. Increasing the photon energy and passing the ionisation threshold one may expect an increase in the ion yield, since the initial absorption leads to emission of an electron. For the N + , NH 2 + , and NH 3 + cations, such an increase of the ion yield above the threshold is clearly seen. Remarkably, almost no increase in the ion yield is seen for NH + (panel b in Fig. 2). This observation may indicate that the mechanisms that lead to creation of N 2+ ions via emission of additional electrons and release of hydrogen atoms differ for the above and below threshold regions for NH + and any other ion in the series NH y + .
The blue dashed lines in Fig. 2 mark the energy region of the atomic nitrogen 1s -2p resonance group. As can be seen in the spectra for the molecular ionic series NH y ( y = 1-3), the respective molecular resonance shifts towards lower energies depending on the number of hydrogen atoms bound to the central nitrogen atom. This is due to increasing hybridisation between the nitrogen valence and hydrogen orbitals. For the atomic nitrogen ion N + , we observe sharp Rydberg lines below and above the lowest 1s ionisation threshold (Fig. 3b), while for the molecular targets the resonance features are broader and overlap with one another. In the following we discuss the assignments of the X-ray absorption resonances for each target NH y + (y = 0-3) in more detail.

Atomic target N +
The ground state of the N + cation is 1s 2 2s 2 2p 2 3 P. Within the experimental ion beam, a non-statistical mixture of several meta-stable terms exists, as has been pointed out before. 36 These various states result in a multitude of multiplet lines as observed in Fig. 3. For comparison of the measured N 3+ ion yield (orange dots in Fig. 3) with the calculated X-ray absorption resonance positions (vertical light blue lines in Fig. 3) we take into account the terms 1s 2 2s 2 2p 2 3 P, 1s 2 2s 2 2p 2 1 D, 1s 2 2s 2 2p 2 1 S, and 1s 2 2s 1 2p 3   higher excitations above 405 eV. Overall, the calculated data show good agreement with the measured ion yield for the energy position of the resonances values below 427 eV. The discrepancies at photon energies above 427 eV are due to the limited capability of the employed basis set to adequatly describe the electronic continuum as discussed in the section Theory Details. For the region below 405 eV, the ion-yield peak heights coincide with the calculated absorption cross section. However, for the region above 415 eV the calculated absorption strength does not always agree with the measured abundance of ions. A particularly drastic disagreement is seen for the 1s 2 2s 1 2p 3 5 S -1s 1 2s 1 2p 3 3p 1 5 P transition at 419 eV. The signal in the measured ion yield is remarkably low (magnified by a factor of 200 in Fig. 3b) in contrast to the relatively strong calculated absorption cross section. The 1s 1 2s 1 2p 3 3p 1 5 P term, e.g., has a particularly high spin that may lead to blocking of certain Auger decay channels, since the spin of most electrons does not match the spin of the core hole. The production of N 3+ ions might therefore be suppressed for this core excited configuration relative to other core excited configurations. Our assignment of the individual resonances for the first resonance region is given in Table 1 and compared with previous calculations using the multi-configuration Dirac-Fock (MCDF) method and the R-matrix method. 36 Most of our results match those of previous calculations, however, there is a slight discrepancy regarding the assignment of the 1s 2 2s 1 2p 3 5 S -1s 1 2s 1 2p 4 5 P transition. In the previous work 36 this line is, according to the R-matrix calculation, associated with the first resonance at 399.8 eV whereas we identify this transitions at C401.5 eV (in agreement with the previous MCDF calculation 36 ).
Overall we conclude that the applied calculation procedure is sufficient to conduct assignment of the resonant transitions.
With our geometry optimisation we obtained a bond distance of 1.065 Å for the 2 P term and of 1.080 Å for the 4 S À term. Fig. 4 shows the calculated absorption resonances (light blue vertical lines) for the NH + cation compared to the measured N 2+ ion yield (dark blue dots). In the figure we used 0.8 and 0.2 as weights for the 2 P and 4 S À terms, respectively. The red dashed line in Fig. 4 is the result of a convolution of the calculated resonances characterised by their positions and absorption oscillator strengths with a 400 meV FWHM Gaussian function for the lower energy part (392 eV to 404 eV) and with 3 eV FWHM Gaussian function for the higher energy part. As for the spectrum of the atomic N + ion, the calculated transition lines have been shifted by 2.1 eV for the region 390 eV to 405 eV and by 1.1 eV for the region above 405 eV.
In the region below 405 eV one can clearly identify four peaks in the experimental spectrum. The weak resonance at 395 eV can be assigned to a 1s 4 S À -3s 4 S À excitation. In the region 397 eV to 400 eV our calculation predicts four excitation resonances 4 S À -4 P, 2 P -2 D, 2 P -2 S À , and 2 P -2 S + . However, in the measured ion yield one can clearly resolve three peaks, only. The calculation suggests that the 2 P -2 D transition is hidden between the 4 S -4 P and the 2 P -4 S À resonances. The remaining resonance peak at 399.5 eV can be associated with a 2 P -2 S À transition.
Above an excitation energy of 405 eV the resonance structures in the measured ion yield are very broad. This might be attributed to the large broadening due to a steep potential energy curve in the final state. The resonance at 411 eV is rather isolated. According to our calculation, the dominant broad peak around 411 eV can mainly be assigned to a 1s -4s ( 2 P -2 P) excitation. Above 415 eV we see the ion yield  decreasing with photon energy, whereas the calculated convoluted absorption cross section suggests rather the opposite trend. The assignments for the dominant transitions in the NH + -XAS are summarised in Table 2.
For the X-ray absorption calculation we also included the closed shell level (1a 1 ) 2 (2a 1 ) 2 (1b 2 ) 2 (3a 1 ) 2 1 A 1 . From geometry optimisation, we obtain an NH bond distance of 1.03 Å and a bond angle of 1501 for the triplet level and a bond distance of 1.05 Å with a bond angle of 1071 for the singlet level. Fig. 5 shows the calculated absorption resonance positions (light blue vertical lines) for the NH 2 + cation compared to the measured N 2+ ion yield (green dots). The orange dashed line is a convolution of the calculated resonance position and strength with a 1.0 eV FWHM Gaussian function. For better comparison, the calculated absorption lines have been shifted by 1.8 eV in the region below 405 eV and by 1.0 eV in the region above 405 eV. Because of the almost linear geometry of the molecular cation, the 1a 1 -3a 1 3 B 1 and 1a 1 -1b 1 3 A 1 excitations are almost degenerate. As can be seen in Fig. 5a, the calculated positions of these two resonances agree with the ion yield peak at 396 eV. The 1a 1 -1b 1 resonance for the tilted geometry of the 1 A 1 cationic level is at distinguishably higher energies. For the higher excitation energies above 405 eV, the observed ion yield peaks can be assigned to calculated absorption lines of the 3B 1 initial level into dipole-allowed higher excited levels of the irreducible representations 3 B 1 or 3 A 2 . Overall good agreement can be seen for resonances up to 420 eV. The assignments are summarised in Table 3.

Molecular target NH 3 +
The neutral NH 3 molecule has a pyramidal geometry and hence its symmetry group is C 3v . 25,27 The cation NH 3 + has a planar geometry and thus belongs to symmetry group D 3h . 61 The electronic ground state of the cation is 1a 0 1 À Á 2 2a 0 1 À Á 2 ð1e 0 Þ 4 1a 00 2 À Á 1 2 A 00 2 . Geometry optimisation gives an NH bond length of 1.024 Å. Fig. 6 compares the calculated absorption lines (light blue vertical lines) for the NH 3 + cation with the measured N 2+ ion yield (red dots). The orange dashed line is a convolution of the calculated resonance position and strength with a 1.0 eV FWHM Gaussian function. The calculated transitions have been shifted by 1.6 eV for the region below 405 eV and by 1.2 eV above 405 eV. Below 405 eV the single ion yield peak must be associated with the 1a 0 1 ! 1a 00 2 2 A 0 1 excitation, as indicated by the calculated transition. Above 405 eV several peaks can be observed in the measured ion yield. These peaks match with those of the calculated transitions into higher excited states up to excitation energies of 420 eV. The assignment of individual resonances is summarised in Table 4.

Chemical shift
As indicated by the light blue dashed lines in Fig. 2, the respective 1s -2p resonances shift to lower excitation energies when going from the atomic cation N + to the molecular cation NH 3 + . For the energies of the 1s -2p resonance peaks, we obtain a systematic sequence of 399.8 eV, 398.8 eV, 396.4 eV, Table 2 Resonance positions E ph and line assignment of the observed resonances in the NH + molecular target ion (see Fig. 4).  395.2 eV. With more hydrogen atoms in the molecule, additional electrons contribute to core-hole screening effects 62 and lower the energy of the core excited state. As discussed previously, 42 this effect leads to a systematic lowering of core excitation resonances. To further confirm this trend, we calculated the ionisation potential of the molecular cations using a D SCF calculation, in which the ionisation energy is obtained from separate SCF calculations for the initial and final electronic states, respectively (see Table 5). As can be seen, apart from the metastable configurations that involve vacancies in 2s/3s orbitals ( 5 S for N + and 4 S + for NH + ), the ionisation potential shifts systematically to lower energies with increasing number of hydrogen atoms. At 400 eV, in Fig. 2 a clear shift of the resonant structures to lower photon energies with increasing number y of H atoms is found for the series of free NH y + ions. This is contrary to shifts found for the alkylamines, 37 where the N 1s resonances are shifting to higher photon energy with increasing number of H atoms.

Absolute count rate for the lower and upper resonance regions
As indicated by the different scale on the calculated cross section axes in Fig. 3-6(a) and (b), the absorption lines for the higher Rydberg region (4415 eV) are weaker than for the first resonance region (390 eV to 405 eV). In contrast, the measured N 2+ ion yield is significantly larger in the Rydberg region compared to the first resonance region. This clearly indicates that the relative probability for creating N 2+ ions is larger for the more highly excited resonances than for the resonances with lower excitation energies. The relative production yields of N 2+ subsequent to excitation into these two resonance regions can be read off the relative change of the ion yield/cross section scale in Fig. 3-6. For N + we observe a factor C5 higher N 2+ production rate, for NH + the factor is C1.8, for NH 2 + the factor is C3.7, and for NH 3 + the factor is C2.3.
Comparison with glycine Fig. 7 shows a comparison between the absorption spectrum of glycine around the nitrogen K-edge 63 and our measured ion yield of NH 2 + . The data of glycine was scaled to match the intensity at energies above the K-edge and the figure is drawn with two different energy axes, because of the different chemical environment of the two molecules. As can be seen, the first Table 3 Resonance positions E ph in and line assignments of the observed resonances in the NH 2 + molecular target ion (see Fig. 5) in its cationic ground level ((1a 1 ) 2 (2a 1 ) 2 (1b 2 ) 2 (3a 1 ) 1 (1b 1 ) 1 3 B 1 ) or excited level ((1a 1 ) 2 (2a 1 ) 2 (1b 2 ) 2 (3a 1 ) 2    resonances below the K-edge are prominent in both spectra, but at higher energies the resonances are much smoother in glycine. The three resonances for glycine are assigned to excitations into virtual orbitals with the local s Ã NH ; p Ã NC ; s Ã NC orbital character. 63 This assignment matches the assignment for NH 2 + attributing the three major resonances mainly to excitations into the 2b 2 , 2b 1 , and 5a 1 orbitals, corresponding to local s Ã NH , p x , and p y orbital character, respectively.
In contrast, the spectra of glycine shows some significant differences compared to the spectra of NH 3 + (see Fig. 6). Thus, this comparison between NH 2 + , NH 3 + and glycine demonstrates that the spectra of ionic fragments can be useful to identify building blocks of more complex molecules.

Conclusions
The N 2+ ion yields of the target molecular cations NH y + (y = 1-3) have been measured in the energy range of the nitrogen K-edge and their absorption resonance structure has been determined.
We have calculated X-ray absorption cross sections based on the configuration interaction method utilising the XMOLE-CULE toolkit. Our calculations show excellent agreement with the ion yield data. For most of the observed transitions, the ion yield seems to correlate with the calculated absorption strength. However, we quantify a trend for a lower abundance of N 2+ ions for excitation in the first resonance region compared to higher resonances. This trend as well as particular deviations (e.g., the very low ion yield for the N + 1s 2 2s 1 2p 3 5 S -1s 1 2s 1 2p 3 3p 1 5 P transition) indicate that the mechanism behind the production of the highly charged ions depends on the specific state populated by the X-ray absorption. In future studies, this effect will be further investigated by comparing the relative ion yields of different fragment charges. The measured ion yield resonances have been identified by the calculated absorption transitions (see Tables 2-4). Depending on the number of hydrogen atoms bound to the nitrogen atom we report characteristic structures in the X-ray absorption. Each additional hydrogen atom and its electron has a significant impact on the spectral features of the different absorption spectra (Fig. 2). These characteristic resonance structures in the NH y + series might help to identify different molecular cationic (sub-)groups within larger molecular complexes such as the amino acid glycine, for example. Nevertheless, the comparison of the free ions NH y + with the amine group in different solutions and molecular environments shows the tremendous influence also of the larger chemical surrounding.