Stark level-crossing spectroscopy of S formaldehyde eigenstates at the dissociation threshold

Abstract

Spectra of S0 D2CO rovibrational eigenstates with 28 300 cm−1 of vibrational excitation are measured by Stark level-crossing spectroscopy. In this new method, the lifetime of a single J, K, M-resolved S1 state is monitored as a function of electric field. Enhanced nonradiative decay causes the S1 lifetime to decrease as S0 states are Stark tuned into resonance. Analysis of the resulting resonance lineshapes yields complete distributions of S0 decay rates (linewidths) and S1-S0 coupling matrix elements. The S0 decay rates represent the first measurements of unimolecular dissociation rates of a polyatomic molecule at the eigenstate-resolved level. S0 decay widths vary from 6.4×10−5 to 3.8×10−3 cm−1 and S1-S0 coupling matrix elements vary from 3.5×10−7 to 4.7×10−5 cm−1, demonstrating that chemical properties of neighboring eigenstates fluctuate by over two orders of magnitude. The observed density of S0 vibrational states is ∼400 per cm−1, six times greater than an estimate including first-order anharmonic corrections. The small increase of level density with J indicates that Ka is nearly a good quantum number for J≤4. The barrier height to unimolecular dissociation on the S0D2CO surface is determined to be 80.6±0.8 kcal/mol, corresponding to 79.2±0.8 kcal/mol for H2CO, in good agreement with ab initio predictions. Quantitative agreement between the magnitude of experimentally determined decay rates and an RRKM rate calculation with all parameters set by ab initio calculation is found.

An intramolecular theory of the mass-independent isotope effect for ozone. I

“…87 It is interesting to compare this behavior with recently studied fluctuations from RRKM behavior, such as in the dissociation of formaldehyde, where the excess energy was produced by optical excitation. 88 Here, the quantum state-dependent rate constants for the dissociation were distributed about the RRKM value, as a function of energy. Had some of the states been nonreactive, the remaining ones, would instead, have been consistently above the RRKM value.…”

Scarring and the Statistics of Tunnelling

“…In this section we review elements of that argument and use them to calculate correlations among the components of x. From this analysis there emerges a natural conjecture for generalising (2). The distributions we calculate on the basis of this conjecture are necessary to understand tunnelling but also, independently of the tunnelling problem, hold promise as a basis for understanding the statistics of scarring in a rather general way.…”

“…With the development of experimental techniques capable of measuring state-specific reaction rates, such tunnelling from highly-excited chaotic states has become directly relevant to the analysis of chemical reactions, for example. Specific examples of reactions which have been treated using RMT-based statistical methods include the dissociation rates of D 2 CO [2,3] and N 2 O [8]. In [11], the distributions arising from such an analysis were shown to be determined in a simple way by the stability and action of a complex tunnelling orbit which crosses the potential barrier with minimum imaginary action.…”

“…Such analyses have been used in particular to understand nuclear resonances [1], chemical reaction rates [2][3][4][5][6][7][8] and quantum dots [9,10]. While the manner in which internal states couple to the continuum varies in these analyses, the internal quantum mechanics is usually modelled using one of the standard ensembles of random matrix theory (RMT).…”

Phase space conduits for reaction in multidimensional systems: HCN isomerization in three dimensions

“…Moreover, as we make clear below, these spherical cylinders bound the true ''phase space reaction paths.'' The ability to compute these manifolds, and to track them far away from the transition region, is the key to the study of state-specific reactivity, 16,17 of dynamical effects on reactions 30,31 of control of reactivity 18,19 and as will be discussed below in more detail, of rare events.…”