The National Science Foundation's (NSF) Tokyo Office periodically receives and disseminates reports on research developments in Japan that are related to the Foundation's mission. NSF-sponsored researchers currently working in Japan prepare many of these reports. These reports present information for use by NSF program managers and policy makers; they are not statements of NSF policy. .

Special Scientific Report #98-30 (November 16, 1998)

The following report was prepared by Dr. Wade Sisk. In January of 1997, Dr. Sisk joined the Molecular Photochemistry Laboratory of the Institute of Physical and Chemical Research (RIKEN) for a two year fellowship sponsored by NSF and the Science and Technology Agency of Japan (STA). Upon completion of his fellowship, Dr. Sisk will return to the University of North Carolina at Charlotte where he is an Assistant Professor of Chemistry. At RIKEN, Dr. Shigeru Ikeda is collaborating with Dr. Sisk. Dr. Sisk can be reached via email at: wsisk@postman.riken.go.jp

Research

I came to RIKEN to work in the molecular photochemistry laboratory of Dr. Hisaharu Hayashi for a two-year NSF/STA Fellowship (Jan. 8, 1997 – Dec. 24, 1998). I was soon placed under the guidance of Dr. Shigeru Ikeda. Our area of research focused on using large magnetic fields to affect the dynamics of gas phase photochemical reactions. Our initial project was extending studies of magnetic field fluorescence quenching of CS₂. Fluorescence quenching the CS₂ 6V and 10V bands under large magnetic fields up to 10 Tesla had previously been studied in this laboratory by Dr. Ikeda. My initial project was to extend those studies to the 13V and 15V bands. My goal was to determine the temporal behavior as well as the rotational and vibrational dependence of magnetically induced fluorescence quenching. In addition, I also worked with Dr. Nilmoni Sarkar and Dr. Ikeda on another project concerning the magnetic field quenching of CSCl₂.

I have presented my results as a poster (3PA35) titled "Fluorescence Quenching of Various CS₂ V System Bands via Large magnetic Fields", at the Molecular Structures Conference, Nagoya, Japan, Oct. 3-5, 1997. I have also presented these results as a poster (#124) titled "Fluorescence Quenching of 13V and 15V Bands of CS₂ via Large Magnetic Fields", at the 216^th Annual American Chemical Society National Conference in Boston, August 23, 1998 within the physical chemistry division. In addition to this I presented my research to our sister university, Tokyo University of Agriculture and Technology, to encourage students to participate in exchange programs. I have also made oral presentations of my research at the 6^th MR Science International Workshop on Chemical Dynamics, Oct. 27, 1997, Hakone, Japan, and the 7^th MR Science International Workshop on Chemical Dynamics, Oct. 31, 1998, Tokyo, Japan. We are in the process of preparing two manuscripts to be submitted for publication concerning magnetic field quenching of CS₂ and magnetic field quenching of CSCl₂.

Abstracts of Manuscripts to Submit for Publication

1. Fluorescence Quenching of Several Vibrationally Excited V Bands of CS₂ via Large Magnetic Fields

Wade N. Sisk, Nilmoni Sarkar, Shigeru Ikeda, Hisaharu Hayashi, Molecular Photochemistry Laboratory, RIKEN

The present investigation examines fluorescence quenching of the u ₂ vibrationally excited 11V, 13V, 15V and 21V bands of gaseous CS₂ by large magnetic fields. Rotational and vibrational dependence of fluorescence magnetic field quenching (MFQ) is obtained from fluorescence excitation spectra at several fields. At high fields quenching for a K´ = 1 band (13V) exceeds that for the corresponding K´ = 0 band (15V), consistent with previous observations of the 6V (K´ = 1) and 10V bands (K´ = 0).¹ This enhanced MFQ is explained via nuclear spin statistics and selection rules (D J ¹ 0 for K´ = 0). A magnetic field dependence could not be established for the vibrational bending quantum number u _2¢ .

In the previous experiment on the 6V and 10V bands, the fluorescence decay profiles were described by a biexponential decay function in which the fast decay component has a lifetime t less than the laser FWHM. In the present bands (u ₂´ > 0), fast components with t < laser FWHM are not observed in contrast with the 6V and 10V bands. Possible explanations for this difference are discussed. For certain rotational lines of the 15V band, field-induced quantum beats were observed in the time-decay profiles. These quantum beats are shown not to be Zeeman beats between M_J or M_S sublevels, but rather molecular beats between the coherently mixed levels from different zero order electronic states. In addition to molecular quantum beats, field-induced dips in fluorescence were observed in the magnetic field scans for the 15V band. These results may be explained by level anticrossing.

2. Effect of Large Magnetic Fields on the Fluorescence from the State of Gaseous Cl₂CS

Nilmoni Sarkar, Shigeru Ikeda, Wade N. Sisk, Hisaharu Hayashi, Molecular Photochemistry Laboratory, RIKEN

The fluorescence excitation spectra and fluorescence decay profiles of the state particularly 4₀¹, 2₀¹4₀¹ and 2₀¹3₀¹4₀¹, bands have been observed under various magnetic fields from 0 – 10 T region. From measurements of fluorescence excitation spectra, it is observed that the intensity of the spectra has been considerably reduced by magnetic fields. The reduction in intensity is 86% for the ³⁵Cl₂CS band of 2₀¹4₀¹ at 10 T and 90% for the 2₀¹3₀¹4₀¹ band at 10 T. The reduction in intensity is only 30% for the 4₀¹ band at 10 T. The fluorescence decays, which are bi-exponential at zero field for these three bands, suggest that the molecule belongs to the intermediate case according to the theory of non-radiative transitions. The change in the ratio fast-to-slow pre-exponential factors, A_f/A_s, with increasing magnetic field is consistent with the change in the quenching ratio. In the case of the 2₀¹3₀¹4₀¹ band, the fluorescence decay changes from bi-exponential to single exponential at high field, which indicates that the molecule changes from intermediate case to the statistical limit. The pressure and field dependence of the quenching ratio indicates that these experimental results are interpreted by the magnetically induced radiationless transition due to the direct mechanism (DM). There is a strong dependence of magnetic quenching on the excited state energy. The field induced acceleration of internal conversion, i.e., DM has shown a discontinuity at 19500± 100 cm^-1, indicates the existence of dissociation limit : .

Future Benefit

I am very pleased to have had this opportunity to carry out this novel research at RIKEN. I plan to continue pursuing this research when I return to my position as assistant professor of chemistry at the University of North Carolina Charlotte. When I applied for the NSF/STA fellowship, I emphasized the merit of my stay in the continuity of research upon my return. In this regard, in August I submitted a $219,000 Defense University Research Instrumentation Program proposal to the Office of Naval Research for instrumentation necessary to extend these experiments in my laboratory at UNC Charlotte. In addition to the DURIP proposal, I also submitted a proposal to NSF in August to provide summer salaries for faculty and students and fund supplies for this project. These two grant proposals seek to fund research on studying photodissociation dynamics of small molecules in large magnetic fields. The technique of magnetic field fluorescence quenching (MFQ) will be utilized in that the photodissociation products will be probed by LIF in the presence of a large magnetic field. By recording the fluorescence excitation spectra of the photofragments, I hope to obtain the nascent rotational and possibly vibrational distribution of the photofragments. This will yield information as to how the dynamics of dissociation are affected by large magnetic fields. This is a first step in the overall effort to utilize magnetic fields to control gaseous chemical reactions.

Comments

American researchers accustomed to a certain amount of autonomy and privacy may be a little surprised by the research environment in Japan. The shared offices do not allow much privacy. Autonomy may not be extended as much as one would desire, perhaps due to how the relationship is viewed between the host researcher and the American research fellow. I viewed my relationship with Dr. Ikeda as that of colleagues. Much to my chagrin, he viewed it more as that of teacher-student. This may be attributed to more deference shown towards those in authority in Japan, than in the United States. In the final analysis, the inconvenience is a small price to bear, for the large returns in scientific research at one of the world’s premier scientific institutes.