Kaoru Yamanouchi

Vibrational level structure of highly excited SO2 in the electronic ground state. II. Vibrational assignment by dispersed fluorescence and stimulated emission pumping spectroscopy

The dispersed fluorescence (DF) spectra for the fluorescence emitted from eight single rovibronic levels in the C 1B2 state are observed. The vibrational quantum numbers for these levels are (v’1,v2,v’3) =(0,0,0), (0,1,0), (0,2,0), (0,0,2), (1,0,0), (1,1,0), (1,2,2), and (3,1,0). The 484 vibrational levels distributed between 4300 and 21 600 cm−1 in the electronic ground state X 1A1 are identified based on the assignment of the 1388 transitions in the DF spectra. The vibrational level energy is expressed by an anharmonic expansion with 19 coefficients and the vibrational quanta for the three normal modes. The combination analysis of the DF and stimulated emission pumping spectra having the same upper rovibronic level clarifies the vibrational level structure in the vibrationally highly excited region above 17 000 cm−1.

Interatomic potentials of A 30+ and B 31 states of HgHe, HgNe, and HgAr van der Waals complexes

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.

Extreme ultraviolet free electron laser seeded with high-order harmonic of Ti:sapphire laser

The 13th harmonic of a Ti:sapphire (Ti:S) laser in the plateau region was injected as a seeding source to a 250-MeV free-electron-laser (FEL) amplifier. When the amplification conditions were fulfilled, strong enhancement of the radiation intensity by a factor of 650 was observed. The random and uncontrollable spikes, which appeared in the spectra of the Self-Amplified Spontaneous Emission (SASE) based FEL radiation without the seeding source, were found to be suppressed drastically to form to a narrow-band, single peak profile at 61.2 nm. The properties of the seeded FEL radiation were well reproduced by numerical simulations. We discuss the future precept of the seeded FEL scheme to the shorter wavelength region.

State‐specific unimolecular reaction of NO2 just above the dissociation threshold

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, ...

Coincidence imaging of Coulomb explosion of CS2 in intense laser fields

Abstract The coincidence imaging technique is applied to direct determination of the momentum vectors of all the fragment ions produced through every event of the Coulomb explosion of a single molecular ion, CS 2 z+ (z=2–4) formed in intense laser fields (∼60 fs, 0.36×10 15 W / cm 2 ). The molecular structure of CS23+ just before the Coulomb explosion, CS23+→S++C++S+, is reconstructed from the measured momentum vectors of the fragment ions. The mean C–S bond length of 〈r〉=2.5 A and the azimuthally averaged S–C–S bond angle, 〈γ〉=145°, thus determined indicate that structural deformation occurs to a large extent in the intense laser fields.

Dissociative ionization of ethanol in chirped intense laser fields

The dissociative ionization of ethanol C2H5OH in an intense laser field is investigated with a chirped laser pulse. From the sensitive dependence of the relative yields of the fragment ions on the absolute values of the linear chirp rate, it is shown that the light-dressed potential-energy surface (LDPES) at the singly charged stage governs the nuclear dynamics, and that the nuclear wave packet flow into the breaking of either of the C–C and C–O chemical bonds could be characterized by the holding time thold during which the LDPESs are maintained. It is also understood in term of the holding time that the enhanced ionization into the doubly charged stage followed by the Coulomb explosion at C–C or C–O proceeds when the nuclear wave packet at the singly charged stage reaches the critical distance for the further ionization.

Laser-induced fluorescence spectroscopy of He-, Ne-, Ar-, and Kr-aniline van der Waals complexes in a supersonic free jet. Analysis of rotational contours

Abstract Laser-induced fluorescence spectra of the binary van der Waals (vdW) complexes of aniline with rare gas atoms (He, Ne, Ar, and Kr) in a supersonic free jet were observed. The rotational contours of their spectra were analyzed to determine the geometrical structures. The vdW bond lengths of the He-, Ne-, and Ar-aniline complexes in the X 1 A′ state are found to be 3.65(10), 3.35(4), and 3.50(4) A, respectively, while those in the A 1 A″ states for the Ne-, Ar- and Kr-aniline complexes are found to be shorter than those in the X 1 A′ state by 0.05(4), 0.08(3), and 0.15(10) A, respectively. The observed frequency shifts of the bands of the complexes relative to that of free aniline are in good correlation with the polarizabilities of the rare gas atoms.

Vibrational level structure of highly excited SO2 in the electronic ground state as studied by stimulated emission pumping spectroscopy

The stimulated emission pumping (SEP) spectroscopy is applied to SO2 cooled rotationally in a supersonic free jet to investigate the vibrational and rotational level structure in the 17 300–17 900, 21 400–21 500, and 22 200–22 500 cm−1 regions of the electronic ground state. It is concluded that respective vibrational levels are found to couple with each other by the network of Fermi and Coriolis interactions on the basis of the following grounds: (1) there are a number of Fermi pairs of the vibrational levels, and (2) the number of vibrational levels identified as the final states of the SEP transitions increases for rotational levels with larger rotational quantum numbers J and Ka. The distribution of the nearest neighbor vibrational level spacing shows that the vibrational quantum dynamics in the observed regions is almost quasiperiodic and the onset of the quantum chaos is estimated to be above 17 900 cm−1.

Interatomic potentials of HgXe van der Waals complex formed in supersonic jets as studied by laser induced fluorescence spectroscopy

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 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).

Experimental and theoretical exploration of photodissociation of SO2 via the C̃1B2 state: identification of the dissociation pathway

Abstract The photodissociation reaction of SO2 via the C1B2 state, SO 2 ( C 1 B 2 ) → SO ( 3 Σ − ) + O ( 3 P ) was investigated by experimental and theoretical approaches cooperatively to clarify its dissociation mechanism. We measured the laser induced fluorescence (LIF) spectrum of the C-X band in the short UV wavelength region (210-200 nm) under jet-cooled conditions. The fluorescence quantum yields and the dissociation rates of individual vibronic levels were determined in the 220-200-nm region using (i) the LIF spectrum measured in the present study, (ii) that measured previously by Yamanouchi et al. (J. Mol. Struct. 352/353 (1995) 541) in the longer wavelength region above 210 nm, and (iii) the high-resolution absorption spectrum measured by Freeman et al. (Planet. Space. Sci. 32 (1984) 1125). The dissociation rates were also derived in the 210-200-nm region from the broadening of the rotational lines of the C-X vibronic transitions. It was found that the dissociation rates determined through two different procedures were consistent with each other, and that the rate increases almost exponentially as an excess energy above the dissociation threshold increases though there is a certain fluctuation of the dissociation rates reflecting a mode specificity. We also performed theoretical ab initio calculations to derive potential energy surfaces (PESs) of the electronic ground states and low-lying electronically excited states of SO2 within the MCSCF and MRCI levels. The theoretical calculations showed that (i) the PES of the 21A′ state (the C1B2 state in C2v symmetry), correlating with the SO ( 1 Δ) + O ( 1 D ) asymptote, crosses with the repulsive singlet (31A′) state, correlating with the SO ( 3 Σ − ) + O ( 3 P ) asymptote, to form a pseudo-seam, (ii) the crossing pseudo-seam of these two PESs is located near the equilibrium bent angles for the X and C states along the energy contour of ∼9700 cm−1 measured from the SO ( 3 Σ − ) + O ( 3 P ) dissociation limit, and (iii) the crossing seam between the 21A′ (C1B2) and repulsive 23A′ states is located in a lower energy region than the singlet seam; at ∼6700 cm−1 measured from the SO ( 3 Σ − ) + O ( 3 P ) dissociation limit. On the basis of the above experimental and theoretical results together with the previous experimental evidence, we propose that (i) the photodissociation reaction via the C state proceeds mainly through the vibronic mixing between the C state vibronic levels with the quasi-bound dissociation continuum of the electronic ground X1A1 state, and (ii) the additional dissociation channels may be open through the crossing seam with the repulsive singlet (31A′) state and that with the repulsive triplet (23A′) state in their narrow crossing energy regions.