Ultrafast Two-Dimensional Infrared Spectroscopy of a Quasifree Rotor: J Scrambling and Perfectly Anticorrelated Cross Peaks

Abstract

Ultrafast two-dimensional infrared (2DIR) spectra of the N_{2}O ν_{3} mode in moderately dense SF_{6} gas exhibit complex line shapes with diagonal and antidiagonal features in contrast to condensed phase vibrational 2DIR spectroscopy. Observed spectra for this quasifree rotor system are well captured by a model that includes all 36 possible rovibrational pathways and treats P (ΔJ=-1) and R (ΔJ=+1) branch resonances as distinct Kubo line shape features. Transition frequency correlation decay is due to J scrambling within one to two gas collisions at each density. Studies of supercritical solvation and relaxation at high pressure and temperature are enabled by this methodology.

Two-Dimensional Infrared Spectroscopy From the Gas to Liquid Phase: Density Dependent J-Scrambling, Vibrational Relaxation, and the Onset of Liquid Character

“…As seen in the figure (and Table 1), rotational equilibration occurs within 1 to 2 collisions over this order of magnitude range of SF6 fluid densities. Thus, as noted previously for the lowest density dense gas phase 2DIR analysis, 6 J memory loss due to collisions remains a highly efficient process in the supercritical region. The efficiency of rotational relaxation will be contrast with that of vibrational relaxation at these state points subsequently.…”

“…is a measure of the spectral diffusion dynamics or equivalently the timescale for J-specific initial transition frequency memory loss, J-scrambling, due to bath interactions. 6 More specifically, for these gas/fluid phase environments, it corresponds to the time for the excited rotational distribution to return to equilibrium as result of collisions of N2O with the SF6 solvent/bath.…”

“…As introduced above, a final aim of these studies is to learn how density dependent 2DIR measurements can reveal and identify the onset of liquid properties in supercritical and near-liquid density solutions and is made possible by our recent characterization of free rotor 2DIR spectra in low density solutions. 6 There is an extensive literature on the observation and molecular-level interpretation of the vibrational spectra of small molecules, especially IR absorption spectra of hydrogen halides in high pressure and simple non-polar liquids, that closely relates to this 2DIR…”

“…5 Given the lack of spectral resolution largely due to quasi-elastic and inelastic collisional effects in such high density/pressure fluids, echo-like spectroscopic techniques, such as 2DIR, are uniquely suited to uncover dynamical information on how excited molecules in these dense environments return to equilibrium with both J and v specificity. 6 Rates of chemical reactions are often controlled by how quickly excited or highly energetic species lose internal energy and return to rotational and vibrational thermal equilibrium. As demonstrated here, 2DIR offers a unique capability for learning about both rotational and vibrational relaxation as a function of fluid density and thus can thus be exploited to provide critical tests of our fundamental theoretical understanding of small molecule relaxation in dense chemical reaction environments.…”

“…3 While all previous ultrafast 2DIR studies spectra have been carried out for vibrators in condensed phase solutions, we have recently described the first corresponding 2DIR analysis of a quasi free quantum rotor. 6 The 2DIR spectra of the resonantly excited n3 asymmetric stretching rovibrational band (2220 cm -1 ) of N2O in SF6 gas at two densities (r* = r/rc = 0. 16…”