Controlling Rotationally Resolved Two-Dimensional Infrared Spectra with Polarization

Recent advancements in infrared frequency combs will enable facile recording of coherent two-dimensional infrared spectra of gas-phase molecules with rotational resolution (RR2DIR). Using time-dependent density-matrix perturbation theory and angular momentum algebra techniques, we derive new polarization conditions unique to freely rotating molecules and absent in the condensed phase. These polarization conditions can be used to suppress parts of 2DIR rovibrational response, clarifying complicated RR2DIR spectra. With the polarization control methods described here, RR2DIR spectroscopy can be a powerful tool for studying complex gas mixtures of polyatomic molecules

Theory of rotationally resolved two-dimensional infrared spectroscopy including polarization dependence and rotational coherence dynamics

Two-dimensional infrared (2DIR) spectroscopy is widely used to study molecular dynamics, but it is typically restricted to solid and liquid phase samples and modest spectral resolution. Only recently has its potential to study gas-phase dynamics begun to be realized. Moreover, the recently proposed technique of cavity-enhanced 2D spectroscopy using frequency combs and developments in multicomb spectroscopy is expected to dramatically advance capabilities for acquisition of rotationally resolved 2DIR spectra. This demonstrates the need for rigorous and quantitative treatment of rotationally resolved, polarization-dependent third-order response of gas-phase samples. In this article, we provide a rigorous and quantitative description of rotationally resolved 2DIR spectroscopy using density-matrix, time-dependent perturbation theory and angular momentum algebra techniques. We describe the band and branch structure of 2D spectra, decompose the molecular response into polarization-dependence classes, use this decomposition to derive and explain special polarization conditions, and relate the liquid-phase polarization conditions to gas-phase ones. Furthermore, we discuss the rotational coherence dynamics during the waiting time

FULLY AB INITIO SECOND-OVERTONE LINE SHAPES OF CARBON MONOXIDE PERTURBED BY ARGON: A COMPARISON WITH EXPERIMENT

We present fully \textit{ab initio} calculations of rovibrational line shapes of carbon monoxide perturbed by argon. The quantum mechanical scattering problem between the CO molecule and the Ar atom is solved numerically for an \textit{ab initio} interaction potential. We use the generalized Hess method to determine the spectroscopic cross sections which describe the effect of collisions on a spectral line\footnote{L. Monchick and L. W. Hunter, JCP 85, 713 (1986); G. Kowzan, P. Wcisło, M. Słowiński, P. Masłowski, A. Viel, F. Thibault, JQSRT 243, 106803 (2020).}. Using these cross sections, we determine line-shape parameters of the Hartmann-Tran profile and the speed-dependent billiard ball profile and generate the line profiles at the experimental conditions of the reference measurements. We compare the generated line shapes with high-quality experimental line profiles\footnote{G. Kowzan, K. Stec, M. Zaborowski, S. W\'{o}jtewicz, A. Cygan, D. Lisak, P. Mas\l{}owski, and R. S. Trawi\'{n}ski, JQSRT 191, 46 (2017).} obtained at five pressures between 0.01 and 1 atm and obtain sub-percent agreement in the whole pressure range. Calculations on the P(9) line are used to test the accuracy of a second Ar-CO interaction potential. The discrepancies between the results for both potentials and the experimental data are explained within the presented theoretical framework. Based on these results and quantum mechanical calculations of mass diffusion cross sections, we clarify the relations between the frequency of velocity-changing collisions and the complex Dicke parameter

Ultrafast internal conversion and photochromism in gas-phase salicylideneaniline

Salicylidenaniline (SA) is an archetypal system for excited-state intramolecular proton transfer (ESIPT) in non-planar systems. Multiple channels for relaxation involving both the keto and enol forms have been proposed after excitation to S$_1$ with near-UV light. Here we present transient absorption measurements of hot gas-phase SA, jet-cooled SA, and SA in Ar clusters using cavity-enhanced transient absorption spectroscopy (CE-TAS). Assignment of the spectra is aided by simulated TAS spectra, computed by applying time-dependent complete active space configuration interaction (TD-CASCI) to structures drawn from nonadiabatic molecular dynamics simulations. We find prompt ESIPT in all conditions followed by the rapid parallel generation of the trans-keto metastable photochrome state and fluorescent keto state in parallel. Increasing the internal energy increases the photochrome yield and decreases the fluorescent yield and fluorescent state lifetime observed in TAS. In Ar clusters, internal conversion of SA is severely hindered but the photochrome yield is unchanged. Taken together, these results are consistent with the photochrome being produced via the vibrationally excited keto population after ESIPT

Ultrafast internal conversion and photochromism in gas-phase salicylideneaniline

Salicylideneaniline (SA) is an archetypal system for excited-state intramolecular proton transfer (ESIPT) in non-planar systems. Multiple channels for relaxation involving both the keto and enol forms have been proposed after excitation to S1 with near-UV light. Here, we present transient absorption measurements of hot gas-phase SA, jet-cooled SA, and SA in Ar clusters using cavity-enhanced transient absorption spectroscopy (CE-TAS). Assignment of the spectra is aided by simulated TAS spectra, computed by applying time-dependent complete active space configuration interaction (TD-CASCI) to structures drawn from nonadiabatic molecular dynamics simulations. We find prompt ESIPT in all conditions followed by the rapid generation of the trans keto metastable photochrome state and fluorescent keto state in parallel. Increasing the internal energy increases the photochrome yield and decreases the fluorescent yield and fluorescent state lifetime observed in TAS. In Ar clusters, internal conversion of SA is severely hindered, but the photochrome yield is unchanged. Taken together, these results are consistent with the photochrome being produced via the vibrationally excited keto population after ESIPT