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Information about the unimolecular acetylene (HC 3/4 CH)↔vinylidene (H2C=C:) isomerization on the S0 energy surface has been extracted from vibrationally unassigned high resolution stimulated emission pumping (SEP) spectra of acetylene. The combination of a new pattern recognition scheme, spectral cross correlation (SCC) with complete nuclear permutation-inversion (CNPI) group theory is shown to be a powerful new technique for characterizing bond rearrangement in highly vibrationally excited normally rigid polyatomic molecules. SCC detects isomerization ‘‘resonances’’ which destroy an approximate vibrational symmetry (e.g., the number of cis-bending quanta). The energies (relative to the zero point level of the stablest isomer) and widths of such resonances provide information about the ‘‘energies’’ of isomer rovibrational levels and the isomer-level-specific isomerization rate. Vinylidene isomerization resonances may be distinguished from ordinary acetylene Fermi or Coriolis perturbations by a unique rotational symmetry dependence due to the correlation between acetylene [D∞h(M)] and vinylidene [C2v(M)] levels in the CNPI group G8. An SCC map of the HCCH S0 15 000–15 900 cm-1 energy region above the zero point level was obtained by comparing SEP spectra recorded via S1(A 1Au)33 and 2162 Ka=1 intermediate levels. The predicted rotational symmetry dependence of the SCC was found between 15 410–15 640 cm-1, but the vinylidene resonance line shape was obscured by Franck–Condon interference effects from well known perturbations between the 33 and 2162 SEP intermediate levels. It was therefore not yet possible to determine the height of the isomerization barrier (or even whether a barrier exists) but an upper bound for the vinylidene zero point level of 15 525 cm-1 was inferred from the SCC data. [ABSTRACT FROM AUTHOR]

The collision-induced deactivation of H2CO in a specific rotational level in its first electronically excited singlet state, A 1A2, v4=1, JKa,Kc=132,12, has been investigated with a series of colliders, M=He, Ar, Xe, and N2. The target level was populated via the transition A←X, 410, 132,12←133,11 using a pulsed dye laser. The subsequent relaxation was monitored using the technique of transient gain spectroscopy (TGS) via the transition A→X, 201301414, 132,12→131,13, which coincides with the line of an Ar+ laser at λ=488.0 nm. The experiments yielded the overall collisional depopulation rate constants (velocity averaged cross sections) for He, Ar, Xe, and N2, respectively: (5.25±0.20)×10-10 cm3/molecule s (39 Å2), (5.22±0.18)×10-10 cm3/molecule s (86 Å2), (5.37±0.29)×10-10 cm3/molecule s (106 Å2), (8.25±0.29)×10-10 cm3/molecule s (125 Å2), where the uncertainties correspond to the 2σ standard deviations. [ABSTRACT FROM AUTHOR]

The high resolution technique of Stark quantum beat spectroscopy is used to examine the electric dipole moment function for the first excited singlet state (A 1A2) of formaldehyde-h2 and formaldehyde-d2. The high precision of these measurements (i.e., better than 5 parts in 104) enables detailed determination of a-axis dipole moment components (μa ) for individual J=2 rovibronic levels in the ν4 out-of-plane bending mode. In the case of 21,1 rotational levels, we find μa (40)=1.4784(7) D and μa (41)=1.4678(4) D for H2CO. For D2CO the measured 21,1 dipole moments are μa (40)=1.4698(6) D, μa (41)=1.4693(3) D, and μa (43) =1.4786(7) D. The state-specific variations in μa revealed by this study reflect the structural influences exerted by the pervasive S1∼S0 nonadiabatic interactions and the pyramidally distorted equilibrium configuration which characterize the A state of formaldehyde. The origin and experimental manifestation of the out-of-plane dipole moment component (μc ) in nonrigid A 1A2 formaldehyde is also discussed. [ABSTRACT FROM AUTHOR]

It is proposed that the two Λ-doublet levels of linear molecules with nonzero electronic orbital angular momentum be labeled Λ(A’) and Λ(A‘), e.g., Π(A’) and Π(A‘) for Π states, etc., according to the following prescription: All series of levels in which the electronic wave function at high J is symmetric with reflection of the spatial coordinates of the electrons in the plane of rotation will be designated Λ(A’) for all values of J, and all those for which the electronic wave function is antisymmetric with respect to reflection will be denoted Λ(A‘). It is emphasized that this notation is meant to supplement, and not replace, the accepted spectroscopic e/f labeling and the parity quantum number. The utility of the Λ(A’)/Λ(A‘) notation is that it is of most relevance in the mechanistic interpretation of reactive or photodissociative processes involving open-shell molecules. [ABSTRACT FROM AUTHOR]

Polarization-detected transient gain spectroscopy (PTGS), a time-resolved PUMP/PROBE scheme with single rotation–vibration level selectivity in both preparative and monitoring stages, has been used to study total depopulation and rotational state-to-state rates in the H2CO A 1A2 v4=1 vibrational level. For H2CO(A)/H2CO(X) collisions, single rovibronic level depopulation rates in the range 88±2 to 127±6 μs-1 Torr-1 (∼10 times faster than the hard sphere gas kinetic rate) were obtained from Stern–Volmer plots which were found to be linear over at least the 0.02–0.2 Torr pressure region. Rotational relaxation between the 50,5 and 60,6 eigenstates reflects a-dipole propensity rules with the sum of first order ΔJ=+1 and ΔJ=-1 processes accounting for well over 50% of the total removal cross section measured for a single rovibronic level. The agreement between collisional decay rates for the 132,12 eigenstate measured previously by the TGS technique and here by PTGS, 110±3 and 106±4 μs-1 Torr-1, respectively, suggests that elastic reorientation processes (ΔJ=ΔKa =ΔKc =0, ΔM≠0) occur with much less efficiency than inelastic energy transfer. Collisional depopulation from single rotational levels of H2CO A 1A2 v4=1 is a remarkably efficient process, but completely understandable in the small molecule, isolated binary collision limit. None of the multiexponential decay and nonlinear Stern–Volmer effects manifest in previous undispersed fluorescence studies remain when PTGS is used to populate and monitor a single rovibronic level. [ABSTRACT FROM AUTHOR]

Individual rotation-vibration levels of the formaldehyde X 1A1 state with 7400

The vibrational spectrumof acetylene is reported. This is the stimulat emission pumping spectrum obtained by stimulated downward transition from an upper electronic state to the ground electronic surface.Broad band and high resolution spectra were recorded at 25840-27250 wave numbers.(AIP) [ABSTRACT FROM AUTHOR]

Collisional energy transfer has been investigated in highly vibrationally excited H2CO (X 1A1) at Evib≅11 400 cm-1 using the method of stimulated emission pumping–transient absorption spectroscopy (SEP-TAS). Total depopulation and state-to-state rate constants were measured for several rotational levels of the 2444 vibrational state. For H2CO self-relaxation the depopulation rate constant of the 61,5 level was measured to be k0=(3.12±0.13)×10-9 cm3/molecule s. An analysis of the state-to-state data with a simplified master equation approach yielded rate constants k1=(7.7±1.2)×10-10, k2=(1.2±0.3)×10-10, and k3=(0.6±0.3)×10-10 cm3/molecule s for collisions with ΔJ=1, 2, and 3, respectively, and ΔKa=0, ΔKc=ΔJ, the reverse rate constants being given by microscopic reversibility. Thus, the ΔJ=±1 steps account for ∼50% of all inelastic collisions, which can be rationalized in terms of a simple dipole–dipole interaction. At Evib≅11 400 cm-1, where ρvib≅0.42/cm-1, collisions seem to conserve the vibrational character in spite of the fact that the energy gap between adjacent vibrational states is a fraction of the transferred rotational energy. [ABSTRACT FROM AUTHOR]