Fast intersystem crossing is observed in the S(1)(1)nπ* state of N-heterocyclic aromatic hydrocarbons and carbonyl compounds. It is attributed to spin-orbit coupling with the (3)ππ* state in the same energy region. The strong singlet-triplet mixing was confirmed by large Zeeman splitting of rotational lines in a high-resolution spectrum. For the S(1)(1)ππ* state of aromatic hydrocarbons, the observed Zeeman splitting was found to be considerably small, and intersystem crossing was considered to be minor. These facts are in accordance with El-Sayed's rule, which states spin-orbit coupling is forbidden between the (1)ππ* and (3)ππ* states. The Zeeman splitting of several derivatives was also observed and the substitution effect on the intersystem crossing rate is discussed.

No abstract

High resolution spectra of the 1 1Π(v′=13–69,J′)←X 1Σ+(v″,J″) and 2 1Π(v′=0–13,J′)←X 1Σ+(v″,J″) transitions of the K8539Rb molecule have been measured with the technique of Doppler-free optical–optical double resonance polarization spectroscopy (OODRPS). Molecular constants of the 1 1Π(v=13–69) and 2 1Π(v=0–13) levels have been determined, and potential energy curves constructed by the RKR method. The RKR potential of the 1 1Π state was found to have a distortion at outer wall, which originates from an avoided crossing of two  1Π states. The perturbations between the 1 1Π(v1,J) and 2 1Π(v2,J) levels were found from the energy shifts of the rotational levels. The magnitude of the nonadiabatic interaction between the 1 1Π(v1=54) and 2 1Π(v2=9) levels, 〈1 1Π(v1=54)|TN|2 1Π(v2=9)〉, was evaluated to be 2.2 cm−1 by a least squares fitting to the energy shifts of the 1 1Π(v1=54,J=20–33) levels. The line intensities were observed to change dramatically around the maximum energy shift. These intensity anomalies are interpreted as an interference effect, which occurs when two interacting levels have comparable transition moments. A remarkable line broadening was observed for the transitions to the 1 1Π(v⩾63) levels, and it was identified as originating from the predissociation to K(4s2S1/2)+Rb(5p2P1/2) atoms. The dissociation energies of the X 1Σ+, 1 1Π, and 2 1Π states have been determined to be 4217.4±0.8, 2021.5±0.8, and 1050.0±0.8 cm−1, respectively.

Fluorescence excitation spectra of CH3CHO, CH3CDO, (CH3)2CO, and (CD3)2CO have been observed in an Ar supersonic nozzle beam. Vibrational analyses have been performed for vibronic bands in the region at wavelengths longer than 313 nm. The 0–0 bands of the S1(n, π*) states were located at 29 771, 29 813, 30 435, and 30 431 cm−1, respectively. The spectra could be analyzed taking the C=O out-of-plane wagging and the CH3 internal rotation as active modes. By fitting a double minimum potential function to the observed vibrational levels, it has been shown that these molecules are pyramidally distorted in the S1(n, π*) state with barrier heights to inversion of 541, 578, 468, and 480 cm−1, respectively. Similar analyses using the Mathieu function gave threefold potential functions for the methyl internal rotation with barrier heights to rotation of 691, 645, 740, and 720 cm−1 for CH3CHO, CH3CDO, (CH3)2CO, and (CD3)2CO, respectively. High resolution measurements of rotational envelopes have shown that the out-of-plane polarization dominates in the acetone spectrum. This result, together with a detailed investigation of the vibronic intensity borrowing mechanism, has led us to conclude that the second order interaction dominates in which the methyl torsion and the C=O out-of-plane wagging are active. The origin of the methyl rotational barrier in the S1 state is discussed on the basis of our recent ab initio calculations. The hyperconjugative interaction is suggested to be important in determining the barrier.

High resolution spectrum of the B 1Π–X 1Σ+ transition of the NaRb molecule was measured with the technique of the Doppler-free laser polarization spectroscopy. Molecular constants of the B 1Π(v=0–12) and X 1Σ+(v=0–6) states of 23Na85Rb were determined. The energy levels of the B 1Π state were found to present many irregularities due to perturbations. The resonance fluorescence spectrum following an excitation to a strongly perturbed level was measured. The fluorescence to the (1)3Σ+ state, which consists of discrete lines followed by a continuum band, was observed in addition to the fluorescence lines to rovibrational levels of the X 1Σ+ state. The perturbing state to the B 1Π(v=8,J=15–21) levels is identified as the (1)3Π state by comparing the observed fluorescence spectra with the selection rules for perturbations and radiative transitions. The dissociation limit of the (1)3Σ+ state, which separates into the Na(3s2S1/2)+Rb(5s2S1/2) atoms, was deduced from the spectrum. The dissociation energies of the X 1Σ+, (1)3Σ+, and B 1Π states were determined to be 5030±2, 182±2, and 1319±2 cm−1, respectively.