Fluorescence characteristics of jet-cooled acetaldehyde at zero field is confirmed to change from thesmall molecule behavior to the statistical limit behavior in CH3CHO and to the intermediate case inCD3CDO, as the excitation energy increases across the dissociation threshold located below 320 nm. Theexcitation energy dependence both of the intensity and of the lifetime of the slow fluorescence showsthat the dissociation rate becomes faster abruptly with increasing excess energy above the threshold, andthe excess energy dependence above the threshold seems to be more drastic in CH3CHO than in CD3CDO.External magnetic field as well as the dissociation in the triplet state plays a role to increase the leveldensity of the triplet state coupled to S1. On excitation below the dissociation threshold, the magneticquenching of fluorescence becomes more efficient with increasing excess energy and the efficiency ofthe quenching is larger in CD3CDO than that in CH3CHO by a factor of about 2. On excitation abovethe threshold, however, the efficiency becomes lower with increasing excitation energy. Thus, the magneticfield effects on fluorescence also change drastically across the dissociation threshold.

Nobuhiro  Ohta

Fluorescence Property and its Magnetic Field Dependence Across thePhotodissociation Threshold of CH3CHO and CD3CDO in a SupersonicJet

Molecular dynamics in organic solution is investigated by the transient grating method with a nanosecondpulsed laser and a fast detection system. First, the dynamics of methyl red (MR) is studied for the probeof the translational diffusion process of solution. The origin of the TG signal which represents the massdiffusion is attributed to the phase grating created by the different refractive indices of the cis and transforms. The cis form of MR, which has a slightly smaller molecular volume than the trans form is foundto diffuse more slowly in alcoholic and aldehyde solutions. The measured diffusion coefficients (D) arecompared with theoretically calculated values. Second, the TG method is applied to elucidate thedynamics of transient radicals created by photochemical reactions. The determined D of the radicals areabout 3&#8764;4 times smaller than those of the parent non-radical molecules even though the molecularvolumes are similar to each other. The advantages as well as the disadvantages of the TG method forelucidating the solvent dynamics are discussed.

Masahide  Terazima

Koichi  Okamoto

Noboru  Hirota

Molecular Dynamics in Solution Probed by the Transient GratingMethod With a Nanosecond Pulsed Laser

The chemiluminescence intensity in the low pressure diffusion flame of the CS2 + O&#47;N2 system wasfound to be considerably affected by external magnetic fields. The emitter in the flame was identifiedas the main emitter in the SO2 afterglow. The measurements of the field strength dependence, collisionaleffect, and spectral distribution of the magnetic field effect revealed the major emitter as the SO2 in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mover accent="true"><mml:mtext>C</mml:mtext><mml:mo>&#x02DC;</mml:mo></mml:mover></mml:math> state. External magnetic fields were also found to quench fluorescence remarkably emitted from thevibronic levels just below the predissociation threshold in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mover accent="true"><mml:mtext>C</mml:mtext><mml:mo>&#x02DC;</mml:mo></mml:mover></mml:math> state. From the observed vibrational-level,field-strength, and pressure dependences of the magnetic quenching, it became evident that the majoremitter of chemiluminescence in the flame could be assigned to the SO2 molecule in the vibronic levelslocated at about 800 cm&#8211;1 below the predissociation threshold in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mover accent="true"><mml:mtext>C</mml:mtext><mml:mo>&#x02DC;</mml:mo></mml:mover></mml:math> state

Haruo  Abe

The Origin of SO2 Afterglows as Revealed by Magnetic Field Effects

A vibronic level study of the spectroscopy and photophysics of the C6H6&#8211;CHCl3 complex has beencarried out using a combination of laser-induced fluorescence and resonant two-photon ionization (R2PI).In C6H6-CHCl3, the S1&#8211;S0 origin remains forbidden while the 1610 transition is weakly induced. Neither610 nor 1610 are split by the presence of the CHCl3 molecule. On this basis, a C3vstructure is deducedfor the complex, placing CHCl3 on the six-fold axis of benzene. The large blue-shift of the complex’sabsorption relative to benzene (+178 cm&#8211;1) and the efficient fragmentation of the complex followingone-color R2PI reflect a hydrogen-bonded orientation for CHCl3 relative to benzene’ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mtext>&#x03C0;</mml:mtext></mml:math> cloud. Dispersedfluorescence scans place a firm upper bound on the ground state binding energy of the complex of2,024 cm&#8211;1. Both the 61and 61 11 levels do not dissociate on the time-scale of the S1 fluorescence andshow evidence of extensive state mixing with van der Waals’ levels primarily built on the 00 level ofbenzene. The C6H6&#8211;(CHCl3)2 cluster shows extensive intermolecular structure beginning at +84 cm&#8211;1, astrong origin transition, and splitting of 61. A structure which places both CHCl3 molecules on the sameside of the benzene ring is suggested on this basis. The vibronic level scheme used to deduce the structureof C6H6&#8211;CHCl3 is tested against previous data on other C6H6&#8211;X complexes. The scheme is found to becapable, in favorable cases, of deducing the structures of C6H6&#8211;X complexes based purely on vibroniclevel data. Finally, the results on C6H6&#8211;CHCl3 are compared with those on C6H6&#8211;HCl and C6H6-H2O to evaluate the characteristics of the n hydrogen bond.

Albert J. Gotch

R. Nathan Pribble

Frederick A. Ensminger

Timothy S. Zwier

The Spectroscopy and Photophysics of <svg style="vertical-align:-0.16379pt" height="8.1499996" version="1.1" viewBox="0 0 10.175 8.1499996" width="10.175" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,7.888)"><path id="x1D70B" d="M574 449q-23 -58 -38 -79q-16 -4 -79 -4q-27 -100 -46 -219q-14 -87 7 -87t74 36l13 -27q-74 -81 -139 -81q-48 0 -48 62q0 22 8 59l60 258l-154 4q-21 -103 -54.5 -209t-64.5 -151q-38 -23 -83 -23l-7 15q46 36 94 152.5t64 216.5q-81 0 -138 -54l-18 23q22 27 39.5 43&#xA;t61.5 33.5t100 17.5q43 0 131.5 -2.5t129.5 -2.5q23 0 33.5 5t24.5 25z" /></g></svg> Hydrogen-Bonded
Complexes: Benzene&#x2013;CHCl3

The excited-state dynamics of small naphthalene clusters that occur upon selective excitation into S1vibronic levels have been investigated by mass-selective pump-probe photoionization spectroscopy. Theresults provide direct evidence for structural isomerization of the initially excited van der Waals clusterinto an excimer geometry, consistent with the fluorescence results. The excimer has been identified asa cluster having a strongly bound dimer core surrounded by ground-state molecules, thus ruling out thepossibility of excimer formation via monomer evaporation. We have also found that ionization is enhancedwhen the initially excited cluster undergoes excimer formation, which provides a new ionizationmechanism for aromatic clusters. The ionization enhancement is explained by the Franck&#8211;Condon factorsassociated with the ionizing transition from the excimer state.

Hiroyuki  Saigusa

Photoionization of Excimers Produced in Naphthalene Clusters

With a state-selective laser excitation, two main decay processes, autoionization and predissociation, invibrational superexcited Rydberg states (n = 8&#8211;12, l = s, p, and f, v = 1) of NO have been studied,directly detecting not only NO+ ions generated by autoionization but also every fragment atom producedby predissociation. In addition, the v = 0 states lying below the ionization threshold have also beeninvestigated with the same method, and the comparison between the two results has more clearlyelucidated the competing behavior of the decay processes in the above-threshold states (v = 1). As aresult, for the v = 1 state, it has been shown that predissociation is the main decay process in the npRydberg states, while autoionization is dominant in the ns Rydberg states. For the nf Rydberg states,the N(4S) + O(3P) predissociation channel, which has not been emphasized in previous studies, has beenfound to play an important role in the decay dynamics both above and below the ionization threshold.

Asuka  Fujii

Norio  Morita

Laser Spectroscopic Investigation of High Rydberg States of no:Decay Dynamics Near the First Ionization Threshold

Mitsuo Ito

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