Dark state vibronic coupling in the Ã(2Π) ←X̃(2Σ+) band of ethynyl radical via high resolution infrared absorption spectroscopy

Abstract

The high resolution infrared spectrum for the à ((2)Π) ← ͠X ((2)Σ(+)) origin band of jet-cooled ethynyl radical (C(2)H) in the gas phase is reported, which exhibits a strong, parity-specific local perturbation in the upper (2)Π(1/2) state. Based on revised parity assignments of the levels, the perturbing state is unambiguously determined to be (2)Σ(+) symmetry, and thus coupled to the à ((2)Π) state by ΔK = ±1 Coriolis interactions. By incorporating Σ-Π Coriolis coupling into the unperturbed Hamiltonian (containing only rotational, spin-rotational, spin-orbit, and lambda-doubling contributions), we are now able to fit the observed (2)Π-(2)Σ(+) origin band to a sub Doppler experimental uncertainty of 15 MHz (0.0005 cm(-1)). In addition, the observation of pairs of transitions to mixed states permits determination of the band origin (ν(pert)) and rotational constant (B(pert)) for the "dark"(2)Σ(+) state, which prove to be in remarkably quantitative agreement with full vibronic predictions of Tarroni and Carter as well as UV dispersed fluorescence studies of Hsu et al. This represents an important benchmark in mapping out non-Born-Oppenheimer vibronic interactions and energy level structure in a polyatomic combustion radical system, an understanding of which will be key to modeling chemical reactions in both terrestrial and astronomical environments.

Theoretical Rovibronic Treatment of the X̃ 2Σ+ and à 2Π States of C2H and the X̃ 1Σ+ State of C2H– from Quartic Force Fields

“…The Ã2Π ← X̃2Σ + origin assignment is assumed to be 3600.4 cm −1 in this study. The location of this band has been supported in many studies 42,44,52 since its first observation from Yan and co-workers at 3600.353(1) cm −1 . 38 Strong agreement for the primary rotational constant for this band is also seen in the same studies 42,44,52 with agreement consistent to three decimal places, 1.405 cm −1 .…”

Infrared laser and Fourier transform spectroscopy of CCH: A highly excited bending vibration of the X̃2Σ+Tokaryk1, Vervloët2, Phi3 2015Journal of Molecular Spectroscopy6180View full textAdd to dashboardCite

“…The Nesbitt group has recently published infrared spectra of jet-cooled CCH at sub-Doppler resolution, which correct some parity misassignments in the bands studied [6]. They also succeed at modelling the interaction between a 2 Π level they observe at 3600 cm −1 and a nearby 'dark' 2 Σ + level [7]. In this paper, we present further new rotationallyresolved spectra of CCH.…”

Spectroscopy, Kinetics, and Dynamics of Combustion Radicals

“…In order to explore these non-adiabatic effects, we have undertaken a spectroscopic reinvestigation of C 2 H under sub-Doppler conditions of the slit jet apparatus. 22,23 At typical rms sensitivity levels of 0.003% in a 10 kHz bandwidth, this translates into signal to noise ratios > 50:1 on the strongest lines for the C 2 H Ã ( 2 Π) X ( 2 Σ + ) band, as shown in Fig. 3.…”

“…The combination of i) slit-jet cooling (T rot ≈ 12 K) and ii) sub-Doppler resolution (≈ 60 MHz) permits satellite branches in each 2 Π 2 Σ + band to be observed and resolved for the first time, helping to clarify parity assignments from previous studies. [22][23][24][25] Of particular dynamical interest is vibronic coupling between the electronically excited à ( 2 Π) state and highly excited vibrational states in the ground state of this radical, which is responsible for the rich complexity observed in the near IR spectrum. To explore this further, a high resolution infrared spectrum of the origin band region near 3600 cm -1 of jet-cooled ethynyl radical (C 2 H) has been explored and analyzed in detail, which exhibits a strong, parity specific rotational crossing in the excited spin orbit F 2 2 Π 1/2 state.…”