Transcript A paradigm shift in the vision of radiationless transitions in

A paradigm shift in the vision of radiationless transitions in
molecules
Groningen: chemistry
Around the nineteen eighties at the University of Groningen, the group of Jan Kommandeur (professor
of physical chemistry from 1963 to 1994) was very active in the study of the interaction between
radiation and matter, in particular, radiation-free transitions in molecules after electronic excitation.
The experimental technique used: fluorescence after excitation aided by lasers. In the group of
Kommandeur these were nanosecond lasers, but elsewhere also picosecond lasers were used. In
particular, the observation of bi-exponential decay was a comprehensive source of investigation both
experimentally and theoretically. This bi-exponential decay had two components, namely a fast and a
slow component.
The slow component still had structure and originated from the normal decay, defined by the lifetime
of the excited state and internal energy conversion. The fast component was, however, attributed to
an in-phase decay of the excited state, linked with near high vibrational states of the ground state and
the first electronically excited triplet state. This rapid component behaved incomprehensible: it
followed the speed of the laser pulse. This meant that with a ns laser, the rapid decline was of the
order of nanoseconds, but with a ps laser decay was of the order of picoseconds.
Nijmegen: Physics
At the time, the Nijmegen group of W. L. Meerts (Molecular and Biophysics) worked hard on the
doubling of visible light to ultraviolet radiation. Woitek Majewski, at that time a postdoc, had managed
to realize doubling within a ring dye laser cavity, with a very high efficiency at that time. Not only the
efficiency was high, also the resolutions was extremely high at that time, with line widths of typically 1
to 10 MHz.
Within the group, Meerts had experience with molecular beams in combination with the high-resolution
UV radiation, which was used to investigate high-resolved UV spectra of, for example, naphthalene.
The time seemed ripe to combine the foregoing, and the group in Nijmegen was helped by chance.
Meerts was invited around February 1982 for the PhD defense of a former Nijmegen student of Meerts,
Van der Meer, who got his PhD from Kommandeur. Meerts invited Van der Meer to come to Nijmegen
with his substance, pyrazine, which happened shortly thereafter.
The experiments went well and Van der Meer could go home after a few days with a wealth of data.
The analysis of the data proved to be very enlightening. Most theories could be trashed because it
could not be proved that there was indeed an interaction between the so-called bright singlet state and
the adjacent dark triplet states. This S-T coupling gave an explanation for the slow decay and its
structure. The fast component was attributed by Jan Kommandeur to a near resonant Raman effect,
which follows the speed of the laser pulse.
Literature:
•
The first article appeared soon after the measurements were completed in November 1982:
(B.J. van der Meer, H.Th. Jonkman, Jan Kommandeur, W. Leo Meerts and Wojtek Majewski.
Spectrum of the Molecular Eigenstates of Pyrazine. Chem.Phys.Lett. 92 (1982) 565-569.)
•
•
•
•
•
•
P.J. de Lange, B.J. van der Meer, K.E. Drabe, J. Kommandeur, W.L. Meerts and W.A. Majewski. The
absolute value of the quantum yield of the B3u; 0-0 state of pyrazine as a function of the rotational
quantum numbers. J. Chem. Phys. 86 (1987) 4004-4010.
W.M. van Herpen, W.L. Meerts, K.E. Drabe and J. Kommandeur. High resolution lifetime
measurements of the perturbed J'=0 state of the 1B3u state of pyrazine. J. Chem. Phys. 86 (1987)
4396-4400.
K.E. Drabe, J. Kommandeur, W.M. van Herpen and W.L. Meerts. Absorption spectrum versus
excitation spectrum of pyrazine. Berichte der Bunsen-Gesellschaft fuer Phys. Chemie 92 (1988)
319-322.
J. Kommandeur, W.L. Meerts, Y.M. Engel and R.D. Levine. The analysis of intensity fluctuation for
a fully resolved spectrum: Pyrazine. J. Chem. Phys.88 (1988) 6810-6813 .
Jan Kommandeur, Wojciech A. Majewski, W. Leo Meerts and David W. Pratt. Pyrazine: An "Exact"
solution to the Problem of Radiationless Transitions. Ann. Rev. Phys. Chem. 38 (1987) 433-62.
W. Siebrand, W.L. Meerts, D.W. Pratt.Analysis and deconvolution of some J not equal 0 rovibronic
transitions in the high resolution S1 <- S0 fluorescence excitation spectrum of pyrazine J. Chem.
Phys. 90 (1989) 1313-1321.
Source: W. Leo Meerts