Impulsive solvent heating probed by picosecond x-ray diffraction

Abstract

The time-resolved diffraction signal from a laser-excited solution has three principal components: the solute-only term, the solute-solvent cross term, and the solvent-only term. The last term is very sensitive to the thermodynamic state of the bulk solvent, which may change during a chemical reaction due to energy transfer from light-absorbing solute molecules to the surrounding solvent molecules and the following relaxation to equilibrium with the environment around the scattering volume. The volume expansion coefficient alpha for a liquid is typically approximately 1 x 10(-3) K(-1), which is about 1000 times greater than for a solid. Hence solvent scattering is a very sensitive on-line thermometer. The decomposition of the scattered x-ray signal has so far been aided by molecular dynamics (MD) simulations, a method capable of simulating the solvent response as well as the solute term and solute/solvent cross terms for the data analysis. Here we present an experimental procedure, applicable to most hydrogen containing solvents, that directly measures the solvent response to a transient temperature rise. The overtone modes of OH stretching and CH3 asymmetric stretching in liquid methanol were excited by near-infrared femtosecond laser pulses at 1.5 and 1.7 microm and the ensuing hydrodynamics, induced by the transfer of heat from a subset of excited CH3OH* to the bulk and the subsequent thermal expansion, were probed by 100 ps x-ray pulses from a synchrotron. The time-resolved data allowed us to extract two key differentials: the change in the solvent diffraction from a temperature change at constant density, seen at a very short time delay approximately 100 ps, and a term from a change in density at constant temperature. The latter term becomes relevant at later times approximately 1 mus when the bulk of liquid expands to accommodate its new temperature at ambient pressure. These two terms are the principal building blocks in the hydrodynamic equation of state, and they are needed in a self-consistent reconstruction of the solvent response during a chemical reaction. We compare the experimental solvent terms with those from MD simulations. The use of experimentally determined solvent differentials greatly improved the quality of global fits when applied to the time-resolved data for C2H4I2 dissolved in methanol.

Synchrotron ultrafast techniques for photoactive transition metal complexes

“…The photo-excited solute relaxes, transferring the energy absorbed from the laser pump pulse to the solvent, which impulsively modifies its thermodynamic state (temperature, pressure and density). As a consequence, the differential scattering intensity is also dominated by the strong time-dependent background related to solvent-only thermodynamic response to laser excitation [43,51]. Nevertheless, such limitations can currently be elegantly overcome by proper analysis and interpretation of the solvent-related contributions.…”

“…A careful modelling of the solvent-solvent contribution is therefore strictly required to isolate structural information on solute (or at least caged-solute) units, embedded in such a huge solvent-related background. After the ultrafast heat release from photo-excited solutes, a first stage of isochoric heating of the solvent can be identified: in this initial phase, the temperature and pressure rise without volume (and thus density) changes [51,52]. According to classical hydrodynamic theory [66], and using typical parameter values found in a time-resolved XSS experiment, this regime can be estimated to last up to approximately 10 ns from laser excitation.…”

“…For this reason, results are often presented in real space, after a sine-Fourier transform operation [43,54,69]. Indeed, a real-space TR-XSS signal represents the differential atom-atom pair distribution function during the course of the reaction [49,51,52,54,70,71]. Positive and negative peaks in real-space curves indicate, respectively, formation or cleaving of chemical bonds at the corresponding interatomic distances.…”

“…Solvent differentials can also be obtained by performing a separate solvent-heating experiment [51], thus bypassing the limitations of the purely computational approach (influence of the selected potential, difficulties in the modelling of the force field related to hydrogen bonds, finite size of the simulation box, etc.). The experimental procedure, first proposed by Cammarata et al [51], consists of the vibrational excitation with near-IR pulses of pure solvent, subsequently probed by 100 ps X-ray pulses at selected time delays. Analysis of the time-resolved data allows one to extract the two key differentials.…”

“…X-rays are scattered by all the atoms included in the probed sample, both solute and solvent atoms. The global scattering signal from a solution can then be decomposed into three contributions [26,[49][50][51][52], schematically indicated in figure 3b.…”