R. Jost

NO2 jet cooled visible excitation spectrum: Vibronic chaos induced by the X̃ 2A1–Ã 2B2 interaction

Significant improvements have been obtained on measurements of the NO2 jet cooled excitation spectrum in the 16 300–18 502 cm−1 range, previously obtained by Smalley et al. [J. Chem. Phys. 63, 4977 (1975)], Persch et al. [Ber. Bunsenges. Phys. Chem. 92, 312 (1988)], and Hiraoka et al. [J. Mol. Spectrosc. 126, 427 (1987)]. The improvements concern first the rotational analysis, owing to a better resolution (150 MHz) and absolute precision (500 MHz), and second the completeness and purity of the resulting vibronic sequence, owing to a better sensitivity. As a result, 159 vibronic energy levels have been observed in the 16 500–18 500 cm−1 energy range, where 210±10 are expected. A detailed comparison with previous results is presented. The statistical analysis of the corresponding energy spacings shows that long range correlations up to 50 mean levels spacings are present, confirming the chaotic behavior of this set of vibronic levels. Furthermore, we analyze the observed rovibronic interactions (or rotation...

Study of the singlet—triplet coupling in glyoxal by level-anticrossing spectroscopy. I. Experimental techniques and results

Abstract Level anticrossing and a new optical-radiofrequency double resonance technique were applied to a study of the singlet—triplet interactions for single rotational levels of the vibrationless 1Au state of glyoxal. The density and spectral distribution of triplet rotational levels virtually coupled to the (K = 6, J = 13) and (K = 7, J = 35) singlet states were determined. The values of the singlet—triplet coupling constants Vst for some selected level pairs were measured. The assignment to the weak-coupling limit is confirmed.

Jet cooled NO2 intra cavity laser absorption spectroscopy (ICLAS) between 11200 and 16150 cm−1

We have combined the high sensitivity of the ICLAS technique with the rotational cooling effect of a slit jet expansion in order to observe and to understand the visible and near infrared NO2 spectrum. By this way, an equivalent absorption pathlength of several kilometers through rotationally cooled molecules has been achieved. Due to the vibronic interaction between the two lowest electronic states, X2A1 and A 2B2, this spectrum is vibronically dense and complex. Moreover, the dense room temperature rotational structure is perturbed by additional rovibronic interactions. In contrast, the rotational analysis of our jet cooled spectrum is straightforward. The NO2 absorption spectrum is vanishing to the IR but, owing to the high sensitivity of the ICLAS technique, we have been able to record the NO2 spectrum down to 11200 cm−1 with a new Ti:sapphire ICLAS spectrometer. As a result 249 2B2 vibronic bands have been observed (175 cold bands and 74 hot bands) in the 11200–16150 cm−1 energy range. Due to the cooling effect of the slit jet we have reduced the rotational temperature down to about 12 K and at this temperature the K = 0 subbands are dominant. Consequently, we have analysed only the K = 0 manifold for N ⩽ 7 of each vibronic band. The dynamical range of the band intensities is about one thousand. Due to the strong vibronic interaction between the X 2A1 and A 2B2 electronic states, we observed not only the a1 vibrational levels of the A 2B2 state but also the b2 vibrational levels of the X 2A1 state interacting with the previous ones. By comparison with the calculated density of states, we conclude that we have observed about 65% of the total number of 2B2 vibronic levels located in the studied range. However, there are more missing levels in the IR because of the weakness of the spectrum in this range. The correlation properties of this set of vibronic levels have been analysed calculating the power spectrum of the absorption stick spectrum which displays periodic motions: the dominant period, at 714 ± 20 cm−1, corresponds to the bending motion of the A 2B2 state. The other observed periods remain unassigned. In contrast the next neighbor spacing distribution (NNSD) shows a strong level repulsion, i.e. a manifestation of quantum chaos. These two observations, apparently contradictory, can be rationalized as follows: the short time dynamics, for t < 10−12 s, is “regular” while for longer times the dynamics becomes “chaotic”. We suggest that this behavior may be observed directly with a pump and probe fs laser experiment.

Study of singlet—triplet coupling in glyoxal by level anticrossing spectroscopy. V. Nature of singlet—triplet interaction

Abstract We observe in glyoxal cooled in a supersonic free jet the fluorescence of individual rotational levels of the S 1 state excited by a cw laser. We use the technique of singlet—triplet magnetic resonance near an anticrossing to measure matrix elements V 31 as a function of rotational quantum numbers N s , N t , K s , K t . The experimental results are compared with theoretical models of singlet—triplet couplings and we show that the spin-vibronic interaction is the dominant singlet—triplet interaction in glyoxal.

Study of singlet-triplet coupling in glyoxal by level anticrossing spectroscopy. VI. Vibrational density and statistics of matrix elements versus energy

Abstract By laser excitation of the rotationless level ( J = 0) of ten vibrational levels of the S 1 (A u ) state (0 0 , 7 2 , 5 1 , 8 1 , 6 1 7 1 , 4 1 , 8 1 7 2 , 2 1 , 8 1 4 1 and 2 1 7 2 ) of supersonic jet cooled glyoxal, we have obtained S 1 -T 1 anticrossing spectra using the homogeneous, high magnetic field (0–8 T) of a Bitter coil. As explained previously, V st is readily obtained from the width of an anticrossing. As triplet vibrational energy increases from 2776 (0 0 of S 1 ) to 4636 cm −1 (2 1 7 2 of S 1 ), the number of anticrossings increases from 38 (0 0 ) to 871 (2 1 7 2 ). The anticrossing density is related to the vibrational density of T 1 . The V st histrograms obtained for each vibrational level are very similar: p ( V st ) ∝ V −1−α st with 0.4 ⩽ α ⩽ 0.7. The more significant and surprising result is that V st > is independent of vibrational energy, even though the corresponding vibrational overlaps predicted a decrease in V st >, of at least two orders of magnitude between 0 0 and 2 1 7 2 . From V st statistics we determine ϱ V st > and ϱ V 2 st > which are the dominant factors for ISC (intersystem crossing). We predict that strong S-T mixing should occur above 6900 ± 500 cm −1 .

Study of singlet-triplet coupling in glyoxal by level anticrossing spectroscopy. IV. Theory of anticrossing position. Application to the measurement of triplet quantum numbers and rotational constants

Abstract We have performed anticrossing experiments in high magnetic fields on several rotational and vibrational singlet levels of glyoxal in a supersonic jet. We show that by analysis of these experiments, we can determine rotational quantum numbers, vibrational symmetry and rotational constants of the triplet level most efficiently coupled to the singlet laser-excited ones.

Study of singlet-triplet coupling in glyoxal by level anticrossing spectroscopy. III. Theory of widths and intensities of double resonances near a singlet-triplet anticrossing. Application to the measurement of relaxation parameters of glyoxal

Abstract We set the theory for the amplitude and widths of double resonance signals near an anticrossing. This theory is used to discuss the possibility of measuring singlet, triplet and coherence relaxation parameters in our experiments on anticrossings between the (K = 6, J = 13) rotational level of vibrationless 0° 1Au state of glyoxal and a sample rovibrational triplet level.

“Gate states” participation in collision-induced intersystem crossing in glyoxal: Experimental evidence

Abstract Collision-induced intersystem crossing in a glyoxal has been studied at room temperature: the presence of gate states participating in this process, i.e. of states presenting a large singlet-triplet mixing coefficient β 2 , has been shown unambiguously. Detailed study of isolated transitions in emission issued from one of such state compared to the so-called “normal state” emission (β 2 ≈ 10 −4 ) is made versus pressure (and/or) magnetic field. This study gives results consistent with a “gate state” mechanism for which theory has been developed recently. It does not, however, rule out the possibility of a non-Wigner singlet-triplet process contribution.

The 3d-triplet complex of molecular hydrogen : the measured Zeeman effect and the calculated eigenvectors and g-factors

An investigation of the triplet 3d-complex of H2, the g(3d) 3Σ g +, i(3d)3Π g , and j(3d)3Δ g states and the nearby h(3s)3Σ g + state, has been undertaken. Approximately 50 g-factors have been measured by optical Zeeman spectroscopy, for the 3d levels for v = 0–3, N ⩽ 7. A concomitant calculation of the L-uncoupling effects and the corresponding eigenvectors in a Hunds case (b) basis has been performed. The accuracy of this calculation is assessed by comparing the experimental g-factors with those predicted by the calculation. For the overwhelming majority of levels, there is good agreement between the predicted and observed g-factors. It is shown that the presence of the h(3s) 3Σ g + state is important for low rotational level (N ⩽ 2) g-factors, but it is considerably less important for higher levels.

S1←S0 laser excitation spectra of glyoxal in a supersonic jet: high-resolution rotational analysis

Abstract The high-resolution rotational spectra of several S 1 ← S 0 vibrational transitions of glyoxal are obtained and analyzed. Undispersed fluorescence excitation spectra are obtained with a CW ring dye laser in a supersonic jet, providing a linewidth down to 100 MHz. Rotational constants and band origins are determined with an asymmetric rotator program. Population distribution in the supersonic jet is studied. The overtones of the torsion mode ( ν 7 ) are examined as an approach to the trans-cis potential barrier. The 8 0 1 and 8 0 1 7 0 2 bands of A + B type, which are too congested at room temperature experiments, are well resolved in this jet experiment. As a result, the rotational constants obtained by fitting low- J and - K rotational transitions are in good agreement with the constants obtained by fitting large- J and - K transitions of room temperature spectra. Furthermore, on about 1000 analyzed lines no rotational perturbation has been observed.