High-Resolution Photoelectron Spectroscopy of Vibrationally Excited Vinoxide Anions

Abstract

High-resolution photoelectron spectra of vibrationally pre-excited vinoxide anions (CH2CHO‒) are reported using the recently developed IR-cryo-SEVI technique. This method is combined with a newly developed implementation of vibrational perturbation theory that can readily identify relevant anharmonic couplings among nearly degenerate vibrational states. IR-cryo-SEVI spectra are obtained by resonant infrared excitation of vinoxide anions via the fundamental C-O (𝜈4, 1570 cm−1) or isolated C-H (𝜈3, 2546 cm−1) stretching vibrations prior to photodetachment. Excitation of the 𝜈4 mode leads to a well-resolved photoelectron spectrum that is in excellent agreement with a harmonic Franck-Condon simulation. Excitation of the higher energy 𝜈3 mode results in a more complicated spectrum that requires consideration of the calculated anharmonic resonances in both the anion and neutral. From this analysis, information about the zeroth-order states that contribute to the nominal 𝜈3 wavefunction in the anion is obtained. In the neutral, we observe anharmonic splitting of the 𝜈3 fundamental into a polyad feature with peaks at 2737(22), 2835(18) and 2910(12) cm−1, for which only the center frequency has been previously reported. Overall, nine out of the twelve fundamental frequencies of the vinoxy radical are extracted from the IR-cryo-SEVI and ground state cryo-SEVI spectra, most of which are consistent with previous measurements. However, we provide a new estimate of the 𝜈5 (CH2 scissoring) fundamental frequency at 1395(11) cm−1 and attribute the large difference with previously reported values to a Fermi resonance with the 2𝜈11 overtone (CH2 wagging).

Spectroscopy of radicals, clusters, and transition states using slow electron velocity-map imaging of cryogenically cooled anions

“…Essentially, the nominal 3=1 anion and neutral levels are mixed with nearby vibrational states, so that the individual vibrational eigenstates are linear combinations of multiple harmonic zero-order vibrational states. A near-quantitative agreement with experiment is obtained using a newly developed implementation of vibrational perturbation theory (VPT) 52,[157][158][159][160][161] that identifies relevant anharmonic couplings among nearly degenerate vibrational states and provides the contributions of the individual harmonic states to the vibrational eigenstate. The underlying concept is illustrated in Figure 16.…”

“…This new experimental configuration, IR cryo-SEVI, has been carried out on a diatomic anion, OHˉ, 51 and, more recently, on the polyatomic vinoxide anion CH2CHOˉ. 52 The OHˉ anion served as a first demonstration of the technique; it is a special case in the sense that the cryo-SEVI spectrum is fully rotationally resolved 51 13 and the gas phase infrared spectrum is well characterized. [131][132] The vinoxide results illustrate the potential and complexity of IR cryo-SEVI, most notably the observation of effects associated with anharmonic coupling between vibrational modes in the anion and neutral as well as the dependence of these effects on the vibrational frequency of the pre-excited mode.…”

“…Quantum chemistry calculations on the vinoxide anion at the B2PLYP-D3/aug-cc-pVTZ level of theory including anharmonicity find two strong infrared transitions, the 4 CO stretch at 1560 cm -1 and the 3 carbonyl CH stretch at 2512 cm -1 . 52 These two modes were thus targeted for our IR cryo-SEVI experiment, with the calculated frequencies taken as starting points for our search. showing a progression in the 9 mode (4 1 0 9 0 𝑛 ), is very similar to the IR-off spectrum, except that the peaks in the IR-on spectrum are shifted to lower eBE by the anion 4 frequency.…”

“…13,50 In addition, a new experimental direction is discussed in which anions are pre-excited with an infrared (IR) laser prior to photodetachment. [51][52] This IR cryo-SEVI experiment probes the effect of vibrational excitation on the anion photoelectron spectrum and provides a new means to measure the vibrational spectrum of a gas phase negative ion.…”

Spectroscopy of Radicals, Clusters, and Transition States Using Slow Electron Velocity-Map Imaging of Cryogenically Cooled Anions

“…13,50 In addition, a new experimental direction is discussed in which anions are pre-excited with an infrared (IR) laser prior to photodetachment. 51,52 This IR cryo-SEVI experiment probes the effect of vibrational excitation on the anion photoelectron spectrum and also provides a new means to measure the vibrational spectrum of a gas phase negative ion.…”