Controlling rotationally-resolved two-dimensional infrared spectra with polarization

Abstract

Recent advancements in infrared frequency combs will enable facile recording of coherent two-dimensional infrared spectra of gas-phase molecules with rotational resolution (RR2DIR). We describe how RR2DIR spectra can be controlled with polarization to enhance or suppress certain features of the spectrum, with new polarization conditions unique to freely rotating molecules and absent in the condensed-phase. 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

“…In this longer article, we aim to provide a comprehensive guide to the theory of rotationally-resolved 2DIR spectroscopy with many example signals. In a shorter companion Letter [28] we highlight some of the key results of this study. Both papers are accompanied by a software package [29] which can be used to simulate rotationally-resolved 2DIR spectra.…”

“…Rotationally coherent pathways unbalance rephasing (S I ) and nonrephasing (S II ) pathways, preventing acquisition of purely absorptive spectra without resorting to the magic angle (MA) condition [28]. This can be explained most easily by comparing S * I with S II pathways shown in Fig.…”

“…In Ref. [28], we present several polarization conditions that improve signal separation in RR2DIR spectra. These conditions and others that suppress specific pathways can be obtained by substituting appropriate c αβ coefficients from Tab.…”

“…In Ref. [28] we present several example of third-order amplitude spectra, where we use polarization conditions with these angles to suppress different subsets of polarization classes.…”

“…In addition to the polarization conditions introduced in Ref. [28], here we introduce two additional conditions: the alternating MA condition, (0, θ MA , 0, θ MA ), and alternating PAC condition, (0, θ PAC , 0, θ PAC ). Addition of these two conditions to the previous four enables total control over polarization-dependent response of diatomic molecules and stretching modes of linear molecules, which are limited to classes Θ 1 , Θ 5 and Θ 6 .…”