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While action spectroscopy of cold molecular ions is a well-established technique to provide vibrationally resolved absorption features, fluorescence experiments are still challenging. Here we report the fluorescence spectra of pyronin-Y and resorufin ions at 100 K using a newly constructed setup. Spectra narrow upon cooling, and the emission maxima blueshift. Temperature effects are attributed to the population of vibrational excited levels in S 1 , and that frequencies are lower in S 1 than in S 0 . This picture is supported by calculated spectra based on a Franck−Condon model that not only predicts the observed change in maximum, but also assigns Franck−Condon active vibrations. In-plane vibrational modes that preserve the mirror plane present in both S 0 and S 1 of resorufin and pyronin Y account for most of the observed vibrational bands. Finally, at low temperatures, it is important to pick an excitation wavelength as far to the red as possible to not reheat the ions.

The vibrational spectroscopy of the water dimer provides an understanding of basic hydrogen bonding in water clusters, and with about one water dimer for every 1,000 water molecules, it plays a critical role in atmospheric science. Here, we review how the experimental and theoretical progress of the past decades has improved our understanding of water dimer vibrational spectroscopy under both cold and warm conditions. We focus on the intramolecular OH-stretching transitions of the donor unit, because these are the ones mostly affected by dimer formation and because their assignment has proven a challenge. We review cold experimental results from early matrix isolation to recent mass-selected jet expansion techniques and, in parallel, the improvements in the theoretical anharmonic models. We discuss and illustrate changes in the vibrational spectra of complexes upon increasing temperature, and the difficulties in recording and calculating these spectra. In the atmosphere, water dimer spectra at ambient temperature are crucial. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Absolute OH-and OD-stretching transition intensities have been calculated for a series of alcohols (methanol, ethanol, 2-propanol, 1-propanol, and tert-butanol) with onedimensional (1D) and three-dimensional (3D) local mode models. We compare the calculated intensities for the Δv OH = 1−5 and Δv OD = 1−3 transitions with experimental values. Potential energy and dipole moment surfaces are calculated at the CCSD(T)-F12a/ VDZ-F12 level of theory. The 1D local mode model includes only the OH(D)-stretching mode, whereas the 3D local mode model also includes the CO-stretching and COH(D)-bending modes. We analyze the effect on vibrational intensities of using either a molecule-fixed Eckart frame or a space-fixed Cartesian frame. We find that both Eckart embedding and inclusion of the COstretching and COH(D)-bending modes in the local mode model are important for the OH/OD-stretching fundamental transition intensities, but have a minor effect on overtone intensities. The 3D reduced-dimensional local model, when combined with coupled cluster surfaces, accurately predicts OH/OD-stretching transition intensities and wavenumbers, for all alcohols included in this work.

The spectroscopy of cis-OSSO and trans-OSSO is explored and put into the context of the Venusian atmosphere, along with other sulfur compounds potentially present there, namely, S 2 O, C 1-S 2 O 2 , trigonal-S 2 O 2 , and S 3. UV−vis spectra were calculated using the nuclear ensemble approach. The calculated OSSO spectra are shown to match well with the 320−400 nm near-UV absorption previously measured on Venus, and we discuss the challenges of assigning OSSO as the Venusian near-UV absorber. The largest source of uncertainty is getting accurate concentrations of sulfur monoxide (3 SO) in the upper cloud layer of Venus (60−70 km altitude) since the 3 SO self-reaction is what causes cis-and trans-OSSO to form. Additionally, we employed the matrix-isolation technique to trap OSSO formed by microwave discharging a gas mixture of argon and SO 2 and then depositing the mixture onto a cold window (6−12 K). Anharmonic vibrational transition frequencies and intensities were calculated at the coupled cluster level to corroborate the matrix-isolation FTIR spectra. The computationally calculated UV−vis and experimentally recorded IR spectra presented in this work aid future attempts at detecting these sulfur compounds in the Venusian atmosphere.