State Mixing and Predissociation in the c̃ ← ã Band System of Singlet Methylene Studied by Optical−Optical Double Resonance †
ABSTRACT In an attempt to characterize the state interactions near the dissociation energy of singlet methylene, the near ultraviolet band system of singlet methylene has been studied using a laser optical-optical double resonance scheme. Spectra terminating in several, previously unobserved, higher bending levels of the c(1)A1 state have been detected. The highest energy band has simple rotational structure with lifetime broadened lines and is observed near 32300 cm(-1), which is 500 cm(-1) above the current best estimate for the singlet bond dissociation energy to CH((2)Pi) + H((2)S). Two lower energy bands exhibit a proliferation of rotationally-labeled double-resonance lines in the vicinity of the bright c(0,12,0) and c(0,13,0) bending levels, indicating that at least 7 and 9 strongly coupled vibronic states participate in each of these bands, respectively. The additional states may be associated with kinks in the adiabatic c state potential along the asymmetric stretching coordinate associated with interactions among c(1)A', a(1)A', and 3(1)A' states, as described by Ostojic (J. Mol. Spectrosc. 2002, 212, 130). There is no evidence for lifetime broadening below the singlet dissociation energy; hence we conclude that coupling of spectroscopically accessible singlet CH2 levels to the triplet manifold is very small.
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ABSTRACT: We report here calculated J = 0 vibrational frequencies for (1)CH(2) and HCN with root-mean-square error relative to available measurements of 2.0 cm(-1) and 3.2 cm(-1), respectively. These results are obtained with DVR calculations with a dense grid on ab initio potential energy surfaces (PESs). The ab initio electronic structure calculations employed are Davidson-corrected MRCI calculations with double-, triple-, and quadruple-zeta basis sets extrapolated to the complete basis set (CBS) limit. In the (1)CH(2) case, Full CI tests of the Davidson correction at small basis set levels lead to a scaling of the correction with the bend angle that can be profitably applied at the CBS limit. Core-valence corrections are added derived from CCSD(T) calculations with and without frozen cores. Relativistic and non-Born-Oppenheimer corrections are available for HCN and were applied. CBS limit CCSD(T) and CASPT2 calculations with the same basis sets were also tried for HCN. The CCSD(T) results are noticeably less accurate than the MRCI results while the CASPT2 results are much poorer. The PESs were generated automatically using the local interpolative moving least-squares method (L-IMLS). A general triatomic code is described where the L-IMLS method is interfaced with several common electronic structure packages. All PESs were computed with this code running in parallel on eight processors. The L-IMLS method provides global and local fitting error measures important in automatically growing the PES from initial ab initio seed points. The reliability of this approach was tested for (1)CH(2) by comparing DVR-calculated vibrational levels on an L-IMLS ab initio surface with levels generated by an explicit ab initio calculation at each DVR grid point. For all levels ( approximately 200) below 20 000 cm(-1), the mean unsigned difference between the levels of these two calculations was 0.1 cm(-1), consistent with the L-IMLS estimated mean unsigned fitting error of 0.3 cm(-1). All L-IMLS PESs used in this work have comparable mean unsigned fitting errors, implying that fitting errors have a negligible role in the final errors of the computed vibrational levels with experiment. Less than 500 ab initio calculations of the energy and gradients are required to achieve this level of accuracy.The Journal of Physical Chemistry A 05/2009; 113(16):4709-21. DOI:10.1021/jp900409r · 2.78 Impact Factor
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ABSTRACT: The origin band in the b̃(1)B(1)-ã(1)A(1) transition of CH(2) near 1.2 μm has been recorded at Doppler-limited resolution using diode laser transient absorption spectroscopy. The assignments of rotational transitions terminating in upper state levels with K(a) = 0 and 1, were confirmed by ground state combination differences and extensive optical-optical double resonance experiments. The assigned lines are embedded in a surprisingly dense spectral region, which includes a strong hot band, b̃(0,1,0) K(a) = 0 - ã(0,1,0) K(a) = 1 sub-band lines, with combination or overtone transitions in the ã(1)A(1) state likely responsible for the majority of unassigned transitions in this region. From measured line intensities and an estimate of the concentration of CH(2) in the sample, we find the transition moment square for the 0(00) ← 1(10) transition in the b̃(1)B(1)(0,0,0)(0)-ã(1)A(1)(0,0,0)(1) sub-band is 0.005(1) D(2). Prominent b̃(1)B(1)(0,1,0)(0)-ã(1)A(1)(0,1,0)(1) hot band lines were observed in the same spectral region. Comparison of the intensities of corresponding rotational transitions in the two bands suggests the hot band has an intrinsic strength approximately 28 times larger than the origin band. Perturbations of the excited state K(a) = 0 and 1 levels are observed and discussed. The new measurements will lead to improved future theoretical modeling and calculations of the Renner-Teller effect between the ã and b̃ states in CH(2).The Journal of Physical Chemistry A 02/2011; 115(34):9440-6. DOI:10.1021/jp1115965 · 2.78 Impact Factor
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ABSTRACT: In this work, we report on our full results of the spectroscopic analysis of the quasi-linear S(2) state of the prototypical halocarbene, CHF, and its deuterated isotopomer CDF using optical-optical double resonance spectroscopy through the S(1) state. A total of 51 S(2) state vibrational levels with angular momenta in the range [script-l] = 0-3 were observed for CHF, and 76 levels for CDF. Progressions involving all three fundamental vibrations were observed, and rotational constants were determined for each of these levels by measuring spectra through different intermediate J levels of the S(1) state. Our experimental results are in excellent agreement with the predictions of vibrational calculations using the discrete variable representation method. The variational vibrational calculations were performed with an analytic potential energy surface fit to ab initio data by the method of interpolating moving least squares. The ab initio data are Davidson-corrected multi-reference configuration interaction calculations based on a state-averaged multiconfigurational self-consistent field reference incorporating a generalized dynamic weighting scheme.The Journal of Chemical Physics 09/2011; 135(10):104315. DOI:10.1063/1.3633724 · 3.12 Impact Factor