ALBERT

Notes: The optical–optical double resonance (OODR) spectra of Rydberg 3Σ+ states of Hg(n3S1)Ne (n=8–10) and Hg(83S1)Ar were measured by using A and B states as intermediate states in the OODR process. The interatomic potentials of three states of HgNe and one state of HgAr were determined over a wide range of interatomic distance, R=3–7 A(ring), by the analysis of the vibrational structure of their OODR spectra. It was found that the potential shape varies sensitively with n and converges to that of the ion core, HgNe+. Dissociation energies (De) of the Rydberg states for the n=8, 9, and 10 were derived to be 209(2), 284(2), and 309(2) cm−1, respectively. Using the quantum defect orbital [G. Simons, J. Chem. Phys. 60, 645 (1974)], which represents a hydrogenic radial wave function for a Rydberg state with a given quantum defect, was introduced to interpret the characteristic n dependence of the interatomic potential. It was shown that the interatomic potential for the Rydberg states can be expressed by the sum of the ion core potential, Vion(R), and the repulsive potential, Vex(R), which originates mainly from the exchange repulsion between the Rydberg electron and the attached rare gas atom. The interatomic potential for Hg(83S1)Ar, whose dissociation energy [De=1602(4) cm−1] is much deeper than that of Hg(83S1)Ne, was also interpreted consistently by expressing the potential as Vion(R)+Vex(R).

Notes: The laser induced fluorescence (LIF) spectrum of jet-cooled NO2 in the energy range from 16 300 cm−1 up to the dissociation limit at 25 130.6 cm−1 was measured with an energy resolution of 0.4 cm−1, and vibronic interaction was discussed through a feature state assignment and a hierarchical analysis. By convoluting the spectrum, the feature states representing bending excited levels in the 2B2 state were identified. The hierarchical level structure just below the dissociation limit was interpreted in terms of a stepwise intramolecular vibronic energy redistribution (IVR) caused by the anharmonic couplings within the 2B2 state followed by the vibronic and rovibronic couplings between the 2B2 and 2A1 states. In a higher resolution (∼0.04 cm−1) measurement the transitions to the rovibronic eigenstates just (0–55 cm−1) below the dissociation limit were resolved. The observed vibronic level density having b2 symmetry, ρvib(b2)=1.6/cm−1, in this energy region is derived from the observed peak density, ρpeak=9.6/cm−1, by assuming a strong K mixing. The observed large peak density was ascribed to the large anharmonisity of the ground state potential energy surface near the dissociation limit. The statistical analyses applied to this eigenstate spectrum showed an extremely strong correlation among these eigenstates, indicating the complete IVR. The present results of the statistical analyses near the dissociation limit support the statistical behavior in the dissociation dynamics just above the dissociation limit investigated in our previous paper [J. Chem. Phys. 99, 254 (1993)].

Notes: The laser induced fluorescence spectrum of jet-cooled XeKr has been measured in the vicinity of the Xe 6s[3/2]01–1S0 atomic transition at 68 045.663 cm−1. The spectrum consists of two band systems, corresponding to transitions to the Ω=0+,1 electronic states from v‘=0 of the ground electronic state. By using the observed band positions and intensities, we have constructed model potentials for both excited electronic states. The Ω=0+ state has a double minimum potential [inner well, re' = 3.09(3) A(ring), De' = 624(3) cm−1; outer well, re' = 5.1(2) A(ring), De' = 101(1) cm−1] while the Ω=1 state potential has only a shallow van der Waals potential [re' = 5.24(4) A(ring), De' = 52.2(7) cm−1]. The double minimum potential for the Ω=0+ state and the difference between the potentials for the Ω=0+ and Ω=1 states are understood in terms of the dominance of two different types of bonding interactions over different ranges of the internuclear distance. At long range, the interaction is dominated by weak dispersion and overlap repulsion between the closed shell Kr atom and the excited Xe atom, giving rise to shallow minima at r≈5 A(ring) in both states. At short range, the XeKr interaction is better described by a XeKr+ ion-core with an excited 6sσ Rydberg electron.The Ω=0+ state is associated with the strongly bound 2Σ+1/2 XeKr+ ion-core, while the Ω=1 state corresponds to the weakly bound 2Π3/2 XeKr+ ion-core. The dual nature of the bonding which gives rise to the double minimum potential in the Ω=0+ state is similar to the bonding seen in excited states of HgAr and HgNe [Duval et al., J. Chem. Phys. 85, 6324 (1986); Okunishi et al., ibid. 98, 2675 (1993); Onda et al., ibid. 101, 7290 (1994); Onda and Yamanouchi, ibid. (submitted)] or the long range s–s, short range d–d bonding seen in the ground state of Cr2 [Casey and Leopold, J. Chem. Phys. 97, 816 (1993)], but is different from some double minima states seen in other diatomics, such as H2 (E,F 1Σ+g) [Davidson, J. Chem. Phys. 35, 1189 (1960); Kolos and Wolniewicz, ibid. 50, 3228 (1968)], Na2 (4 1Σ+g) [Tsai et al., J. Chem. Phys. 101, 25 (1994)], and Cl2 (1 1Σ+u) [Yamanouchi et al., Chem. Phys. Lett. 156, 301 (1989); Tsuchizawa et al., J. Chem. Phys. 93, 111 (1990)] which arise from curve crossings between ionic and covalent diabatic states. © 1994 American Institute of Physics.

Notes: Rotational structures of A–X and B–X vibronic transitions of HgHe, HgNe, and HgAr van der Waals (vdW) complexes formed in supersonic free jets have been investigated. An analysis of the rotational contours shows that rotational structures for the six isotopic species, mHgHe, mHgNe, or mHgAr (m=204,202,201,200,199, and 198), are overlapped, and the observed isotopic splittings are utilized for the definite assignment of the vibrational quantum numbers in the A and B states. Based on the vibrational level spacings and the rotational constants, interatomic potentials for the A and B states of HgNe and HgAr are determined with good accuracy. In the case of the B state of the 199HgRg and 201HgRg (Rg=Ne or Ar), magnetic dipole hyperfine splittings are observed and analyzed.

Notes: The laser induced fluorescence spectra of HgXe formed in a supersonic free jet of a He/Xe/Hg mixture were observed to determine the interatomic potentials between Hg and Xe in the X 10+, A 30+, and B 31 states. The dissociation energies for the X, A, and B states are 254, 1457, and 172 cm−1, respectively, with the equilibrium interatomic distances of 4.25, 3.25, and 4.47 A(ring) for the respective states. It is found that the potential shape for the X and B states can be represented in good approximation by a Morse function, though this function may not reproduce the potential for the A state especially in the vicinity of high vibrational levels (v′〉16).

Notes: The van der Waals (vdW) complexes consisting of benzonitrile and various partner species were formed in a free jet and their laser-induced fluorescence (LIF) spectra were recorded. For all the species chosen as partners (Ar, Kr, N2O, CF3H, and H2O), the LIF spectra showed a red shift relative to that of benzonitrile monomer. The spectral shift increased with increasing dipole moment of the partner species owing to the large dipole–dipole interaction between the partner species and benzonitrile whose dipole moment amounts to 4.14 D. With the aid of computer simulation, the rotational contours of the LIF spectra of the benzonitrile dimer and benzonitrile–Ar complex were analyzed. The dimer was found to be in planar form with the two CN groups facing each other in an antiparallel geometry, whereas in the Ar complex the Ar atom lies over the benzene ring slightly leaning toward the CN group.

Notes: The ν1 band of the CCO radical in the X˜ 3Σ− ground electronic state has been observed in the gas phase by diode laser kinetic spectroscopy. The CCO radical was generated by the 193 or 248 nm excimer laser photolysis of carbon suboxide. By fixing ground state parameters to the microwave values, the band origin and the vibrational changes of the rotational (αB=B0−B1) and spin–spin interaction (αλ=λ1−λ0) constants have been determined to be 1970.864 34(95), 0.003 075 4(85), and 0.008 3(12) in cm−1 with 2.5 standard errors in parentheses.

Notes: Zeeman quantum beats were observed in the fluorescence from a number of rovibronic levels of SO2 in the A˜ 1A2 state. Rotationally cooled SO2 was prepared by supersonic expansion of its seeded mixture in Ar and was excited to the first electronically allowed 1A2 state by a frequency-doubled dye laser in the wavelength range 260–320 nm. In the time-resolved fluorescence measurements, a number of rovibronic lines were found to exhibit beating fluorescence decays in the presence of a weak magnetic field. The beating phenomena were analyzed on the basis of Zeeman quantum beat theory; of particular concern are the oscillation amplitude of the beating and the g value. It was found that almost all rotational levels of upper vibronic states of various bands (D, F, L, and N in Clements' notation) possess sizable magnetic moments. On the other hand, only a limited number of rovibronic lines of several less perturbed bands (E and G) showed beating fluorescence decays. The derived g values and fluorescence lifetimes distribute rather irregularly over a wide range, but an almost linear relationship between the g value and the lifetime was found for the rR0(0) lines of various vibronic transitions. The intramolecular coupling mechanism needed to explain the anomalous behaviors of these excited rovibronic states are discussed. It is suggested that the dominant mechanism is spin-orbit coupling.

Notes: A microwave spectroscopic method has been developed to study elementary reactions in real time through in situ observation of rotational spectra of reaction intermediates such as free radicals with lifetime as short as 1 ms. This method was applied to the O(3P)+ethylene reaction in order to assess the roles of (a) vinoxy+H and (b) CH3+CHO channels in the initial process. The reaction was initiated by irradiating an N2O/C2H4 mixture containing a trace amount of mercury with the 253.7 nm mercury resonance line, and the time evolution of vinoxy, HCO, and H2CO was followed by measuring their microwave absorption intensities as functions of time. The branching ratio was thus determined to be 0.4±0.1 and 0.5±0.1 for (a) and (b), respectively, at the sample pressure of 30 mTorr. The present result agrees with those obtained by Hunziker et al. [J. Photochem. 17, 377 (1981)] using much higher pressures of samples, but is not compatible with the observation of Buss et al. [J. Photochem. 17, 389 (1981)] that (a) is dominant in collision-free conditions.

Notes: Photofragment excitation (PHOFEX) spectra of NO2 are observed by monitoring the specific quantum states of a fragment NO (2Π1/2;v=0, J=0.5–6.5) in the energy region 0–160 cm−1 above the dissociation limit to NO (2Π1/2) and O (3P2). Preparation of NO2 in a quasibound eigenstate above the dissociation limit is attained by the combination of extremely cooled (∼1 K) parent NO2 in a supersonic jet and a high resolution (∼0.05 cm−1) photolysis laser. The dissociation rate constants are obtained from the peak width of PHOFEX spectra and the smallest rate constant is k=8.5×109 s−1, in the energy region where only J=0.5 of NO (2Π1/2; v=0) is produced. The observation that the rate constant increases stepwise when a new product channel J=1.5 opens implies that the transition state is a loose complex. This behavior of the rate constant is direct experimental proof of the statistical theory of the unimolecular reaction process. The product state distribution of NO fluctuates depending on the quasibound state of NO2, though the average value is consistent with the calculation by phase space theory. This state specificity of the rate constant is interpreted in terms of quantum fluctuations associated with individual quasibound eigenstates.