Ultrafast chemical dynamics
Klaas
Wynne
*a and
Neil T.
Hunt
*b
aSchool of Chemistry, WestCHEM, Glasgow University, UK. E-mail: klaas.wynne@glasgow.ac.uk
bDept. of Physics, University of Strathclyde, Glasgow, UK. E-mail: neil.hunt@strath.ac.uk
Abstract
The application of ultrafast femtosecond spectroscopy to chemical dynamics used to be dominated by complications associated with the equipment. The invention of the colliding-pulse mode-locked (CPM) laser in the 1980s at least made femtosecond pulses more commonplace. By contrast however, the arrival of the Ti:sapphire Kerr-lens mode-locked lasers and associated regenerative amplifiers in the 1990s made femtosecond spectroscopy almost “easy”. These lasers provided the added advantage of enabling the generation of femtosecond pulsed radiation at wavelengths ranging from millimetre-wave terahertz radiation, through the infrared and visible, to the deep ultraviolet and beyond, and have shifted the focus of the research largely from the equipment to the chemistry or physics being studied. This wide variety of femtosecond lasers is now being applied to a huge range of problems associated with chemical dynamics and structure, as can be seen in this themed issue of PCCP on Ultrafast Chemical Dynamics.
A number of papers on ultrafast photochemistry are featured in this themed issue. For example, Mathies and co-worker (DOI: 10.1039/c2cp23468h) show that the powerful femtosecond stimulated Raman spectroscopy (FSRS) technique may be used to follow the femtosecond trans to cis isomerisation of an azobenzene derivative. FSRS provides a femtosecond resolved Raman spectrum, which was used to elucidate mechanistic details of the isomerisation process. Other contributions show that ultrafast internal conversion of a free radical releases 3 eV of electronic energy in a remarkably short time of less than 200 fs (Vauthey et al., DOI: 10.1039/c2cp23577c) while isolated cyclopropenylidene and its chlorinated variant exhibit ultrafast 50 fs and 200 fs non-radiative decays (Mestdagh et al., DOI: 10.1039/c2cp23728h). Also featured are a study of the long-standing problem of cis-stilbene isomerisation (Tahara et al., DOI: 10.1039/c2cp23959k) and a UV-pump IR-probe study of photo-induced bond-cleavage of an iron-(III) azido complex (Vöhringer et al., DOI: 10.1039/c2cp23435a).
A number of papers are included employing the technique of two-dimensional infrared (2D-IR) spectroscopy, which was originally developed by Robin Hochstrasser who in this issue presents data on the oxalate ion (DOI: 10.1039/c2cp23892f). The 2D-IR spectra reveal that ion pairs form in caesium oxalate solutions giving rise to anomalously slow (4 ps) relaxation dynamics with interconversion between end-on and side-on conformers on a picosecond time scale. A dramatic illustration of the power of the 2D-IR technique is its application to two anomers of a sugar (differing by a single stereochemical carbon) where the linear infrared spectra are identical but the 2D-IR spectrum shows significant differences (Rubtsov et al., DOI: 10.1039/c2cp23245f). 2D-IR can identify different orientations of side groups and even differences in ultrafast energy relaxation dynamics of certain vibrational modes. The group of Hamm and co-workers applied 2D-IR to the OH (and OD) stretch of water ice Ih, time resolving exciton dynamics and vibrational relaxation (DOI: 10.1039/c2cp23710e). 2D-IR has also been used to determine the origins of equilibrium dynamics near the haeme binding site of nitrosylated ferric myoglobin using a comparison of wild-type protein with a single point mutation (Adamczyk et al., accepted for publication; due to be published online in the near future).
As chemical reactions most often take place in solution, femtosecond liquid and solvation dynamics are of critical importance. An IR pump–probe study in this issue (Tominaga et al., DOI: 10.1039/c2cp23647h) reveals the existence of a complex between acetate and a water molecule giving rise to a rapid quantum beat (with a frequency of ∼80 cm−1) in the time domain signal. An IR pump–probe study by Cho and co-workers (DOI: 10.1039/c2cp23749k) on concentrated KSCN solutions focuses on the concentration dependence of the rotational relaxation time of the SCN− anion and its interpretation in the context of the Hofmeister series, which is also addressed in a paper on the orientational relaxation of water in salt solutions (van der Post & Bakker, DOI: 10.1039/c2cp23882a). Very unusual is an IR pump–probe study of OH/OD relaxation dynamics in a so-called “water bridge” that forms between two beakers of water when a strong DC electric field is applied (Piatkowski et al., DOI: 10.1039/c1cp22358e). The paper by Meech and co-workers (DOI: 10.1039/c2cp23806c) shows that the femtosecond optical Kerr effect (OKE) technique is superior in determining terahertz vibrational spectra over a wide frequency range, here applied to aqueous solutions of amphiphiles. A theoretical analysis by Hynes (DOI: 10.1039/c2cp23555b) shows how vibrational OH-stretch energy is redistributed on a femtosecond timescale in liquid water between the bend, rotations, and translations. Stratt et al. (DOI: 10.1039/c2cp24127g) analyse the theory of changes in the terahertz spectrum induced by an excited molecule.
Studies of the ultrafast dynamics in biomolecules such as photoactive proteins have a long history. Here, the photophysics of DNA is addressed by studies of the decay through internal conversion of UV-excited cytosine polymers by using picosecond time-resolved infrared spectroscopy (Quinn et al., DOI: 10.1039/c2cp23774a) and the relaxation of imidazole in the gas phase by time-of-flight spectroscopy (Ullrich et al., DOI: 10.1039/c2cp23533a). The Kobayashi group has developed a laser system producing 8.7-fs deep UV (around 270 nm) laser pulses, which have been applied to study the electronic and vibrational relaxation dynamics of thymine in aqueous solution (DOI: 10.1039/c2cp23649d). The very short pulses in this case can be used to excite Raman active modes in the excited and ground state. A sub-20-fs pump–probe study of the solvent dependence of S2 → S1 relaxation in the naturally occurring carotenoid spheroidene, shows that an intermediate electronically excited state (whose existence is highly controversial) is shifted in or out of the relaxation pathway and may be populated in about 30 fs (Cerullo et al., DOI: 10.1039/c2cp23585d). Time-domain terahertz spectroscopy has been used by Markelz et al. to observe a dynamical transition in hydrated proteins at 220 K DOI: 10.1039/c2cp23760a).
A small number of papers in this issue address the application of femtosecond lasers to materials. For example, femtosecond UV laser pulses can be used to generate shock waves that propel plumes of neutral biomolecules such as phenylalanine into the gas phase for further study (Greenwood et al., DOI: 10.1039/c2cp23840c). Such molecules do not have any associated solvent molecules. Two papers (Lanzani et al., DOI: 10.1039/c2cp23917e and Elles et al., DOI: 10.1039/c2cp23602h) describe the use of femtosecond pump–probe spectroscopy to study dynamics in conducting polymers.
Development of new ultrafast technology can still open avenues for novel research. The current technological frontier is the generation of femtosecond X-ray and electron pulses for time-resolved diffraction studies. In this issue, it is shown that femtosecond visible-pump and picosecond X-ray probe can be used to study dynamics in two polymorphs of a spin-crossover molecular complex (Buron-Le Cointe et al., DOI: 10.1039/c2cp23587k). It is shown that there is a very good correlation between time-resolved optical data (mainly sensitive to the electronic molecular state) and X-ray data (sensitive to structural deformation) but also that one has to take into account shock-wave driven transformations and heating effects. Finally, Elsaesser and co-workers (DOI: 10.1039/c2cp24072f) present a study of photo-induced mode-softening in an ionic crystal observed with time-resolved X-ray diffraction.
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