Observation of the c̃A11 state of methylene by optical-optical double resonance

Abstract

We report the observation of the rotationally resolved spectrum of the c1A1 state of CH2 via sequential single-photon absorptions at visible and near-infrared wavelengths. Direct absorption from the lowest singlet state a1A1 to c1A1 occurs in the near UV, but it is weak because it corresponds to a two electron transition between the dominant single configuration approximations to the electronic wave functions. Some absorption lines in the c-a system were originally reported in 1966 [G. Herzberg and J. W. C. Johns, Proc. R. Soc. London, Ser. A 295, 107 (1966)], but the weak spectra could not be assigned at the time. Interest in the c1A(1) state was rekindled by recent ab initio results [S. N. Yurchenko, P. Jensen, Y. Li, R. J. Buenker, and P. R. Bunker, J. Mol. Spectrosc. 208, 136 (2001)] which prompted the present work. The new spectra provide accurate energies for rotational levels in the v2linear =11,l = 1 level of the state, and permit assignment of most of the line positions measured by Herzberg and Johns. The double-resonance technique used may be easily extended to the measurement of lower rovibrational levels in the electronic state and possibly also to access the d1A2 state which is theoretically expected to lie at similar energies but, for symmetry reasons, is not accessible from the lowest singlet state in a single electric-dipole transition.

Observation of the state of CH2 by optical–optical double resonance

“…These new spectroscopic measurements employ the same double resonance scheme used previously to characterize thec 1 A 1 ð0; 11; 0Þ levels [14]. We are able to confirm two of the tentative assignments made by Yurchenko et al [15] of the original measurements by Herzberg and Johns [2] and add several more l(K a ) = 0 levels to those now measured precisely.…”

“…Experimental spectroscopic studies have lagged behind theory in this area and only the next highest ðc 1 A 1 Þ state has been observed. Although weak transitions in the ultra-violet region were tentatively ascribed to unknown bands of thec 1 A 1 ã 1 A 1 system by Herzberg and Johns, [2] it was not until very recently that spectroscopic assignments could be confidently made [14] following theoretical work by Yurchenko et al [15].…”

“…[14] A pulsed 308 nm excimer laser created the singlet CH 2 radicals via photolysis of ketene diluted in inert gas in the absorption cell. After a 0.4 ls delay to allow for collisional thermalization, a pulsed OPO laser (Spectra-Physics MOPO-HF) pumped a known rovibronic transition in the CH 2b Àã band system.…”

Rotationally resolved spectrum of the band of HCBr

“…Transitions to this state from theX 1 A 0 will obey parallel rotational selection rules. To our knowledge, theB 1 A 0 state has not been experimentally detected in any halomethylene, but rovibronic transitions involving the correspondingc 1 A 1 state of CH 2 have very recently been detected [23]. In order to investigate this further, we have calculated the relative energies of the low-lying states of HCBr, including theB state, at the MRCI/cc-pVTZ level using the MOLPRO electronic structure package [24].…”

Sub-Doppler spectroscopy of mixed state levels in CH2

“…7 Complex interactions among the electronic states and their mutual perturbations make the assignment of the irregular experimental spectra challenging. A variety of laser techniques have been employed to investigate the b − ã system, including laser induced fluorescence, 10-12 stimulated emission pumping, 13,14 transient absorption, [15][16][17][18][19] and double-resonance saturation spectroscopy [20][21][22] at wavelengths from the nearinfrared to ultraviolet. The energy separation ⌬ ST between the X 3 B 1 state and the ã 1 A 1 state has been determined to be 3147Ϯ 5 cm −1 , 8 and the origin of the b 1 B 1 state lies approximately 8353 cm −1 above the vibrationless level of the ã 1 A 1 state.…”