Average singlet–triplet coupling properties of biacetyl and methylglyoxal using quantum beat spectroscopy

Abstract

Magnetic field effects on the singlerovibroniclevel fluorescence of S 1(1 B 1) pyrimidine: Study of the singlet-triplet coupling by level anticrossings and quantum beats Laserinduced phosphorescence spectroscopy in supersonic jets. The lowest triplet states of glyoxal, methylglyoxal, and biacetylWe have observed quantum beats in the reversible intersystem crossing of simple a·dicarbonyls, and we have analyzed them to obtain information concerning the density of interacting states and the average intramolecular coupling energy. We have analyzed most of the biacetyl quantum beats using a method based on the properties of random matrices which is described elsewhere and we present the results here. We also present an analysis based on perturbation theory using the Fourier transforms of the quantum beats which is appropriate for understanding the methylglyoxal quantum beats. The density of vibrationally hot triplet states which interact with excited singlet states that are connected to the ground electronic state via optical excitation (-22 000 cm -I) is found to increase with the amount of vibrational excitation in the accessible singlet state at roughly the same rate as the overall density of triplet vibrational stress increases. We find satisfactory agreement between the density obtained from the quantum beats and that calculated using well known analytical formulas and direct state counting. The density of interacting states increases with the rotational quantum number of the initially excited singlet rovibronic state. The average value of the magnitude of the spin-orbit interaction is 1-10 MHz independent of the amount of vibrational-rotational excitation present. Radiationless transitions in these highly excited molecules are evidently not subject to any overriding selection rules other than spatial symmetry, and conservation of total energy, total angular momentum, and nuclear spin. The biacetyl quantum beats are collisionally quenched with helium at the same rate as the overall fluorescence. The cross section for this process is roughly 300 ± 200 A 2 • 106

Collision Relaxation Cross Section of Highly Vibrationally Excited Molecules

“…A coherent excitation of the two eigenlevels induces quantum interference which results in an oscillation of the zeroth-order wave functions in time. A well-known quantum interference phenomenon is the fluorescence quantum beat which is induced through the excitation of the bright state, but its decay depends also upon the dark state properties and can be used for the characterization of the dark state.Most quantum beats observed for molecules originate from the coupling between an excited singlet state and a triplet state and have been used for studying the triplet or nonzero spin states [6][7][8][9]. But a few cases related to transitions in the SO 2C √X bands have been determined to involve highly vibrationally excited levels of the electronic ground state.…”

“…Most quantum beats observed for molecules originate from the coupling between an excited singlet state and a triplet state and have been used for studying the triplet or nonzero spin states [6][7][8][9]. But a few cases related to transitions in the SO 2C √X bands have been determined to involve highly vibrationally excited levels of the electronic ground state.…”

Twist dynamics of 9-(N-carbazolyl)-anthracene: effects of Intramolecular Vibrational Redistribution and non-adiabatic transitions in coupled bright and dark states

“…Similar cases of restricted IVR and, therefore, approximately coherent vibrational dynamics in selective degrees of freedom have been discovered previously in other large molecules, including e.g. the phenomena of selective vibrational quantum beats in anthracene [31,32], glyoxal [33] and pyrazine [34], see also [35][36][37]. The resulting message for the femtosecond chemist is that he should be able to monitor the selective quasi-coherent torsional motion of C9A, including also some signatures of the diabatic transitions between the coupled bright and dark states, but rather little can be deduced from the corresponding fs/ps pump-probe-spectra for the competing IVR processes.…”

The role of rotational relaxation in the intersystem crossing between a triplet and a singlet electronic state

“…The involvement of rotation in electronic radiationless transitions (and in particular ISC S 1 → T 1 ), in contrast to the present case of ISC T 1 → S 0 as well as IVR, has been a topic of considerable interest in the recent years18–36. The interest was aroused by two main types of experimental observations—dynamical [measurement of decay rates and emission quantum yields (QYs)] and high‐resolution spectroscopy (probing the dark molecular eigenstates found within the contour of a decaying bright rovibronic level).…”