Fakultät für Chemie und Pharmazie
Institut für Physikalische und Theoretische Chemie
Lehrstuhl für Physikalische Chemie

Research: Molecular Spectroscopy and Photochemistry of Isolated Cold Molecules in the Gas Phase

Prof. Dr. Bernhard Dick
Group Members:

AOR Dr. Uwe Kensy
Dipl. Chem. Christian Greil
MSc. Manuel Schneider
MSc. Nicole Berner

Former Group Members:
Dr. Roger-Jan Kutta
Dr. Andreas Schmaunz

Dipl. Chem. Andreas Wenge
Dr. Angela Kessler

Dr. Thorsten Obernhuber
Dr. Reinhold Seiler
Dr. Robert Aures


Photodissociation dynamics of nitroso compounds

Since NO is a stable radical, the X-N bond in nitroso compounds X-NO is usually weak and can be broken by thermal or photochemical excitation. We cool X-NO in a supersonic jet to very low temperatures (< 2 K). Excitation with a narrow bandwidth laser provides the molecule with a well defined energy. The excess energy of the fragmentation is distributed among the various degrees of freedom of the two fragments. The NO fragments are subsequently analyzed by velocity map imaging (VMI) or 3D-REMPI spectroscopy (see below). In this way we obtain - for every ro-vibrational state of NO in both electronic spin orbit states - the distribution of the velocities and their anisotropies. Thus we know for every state of NO the corresponding kinetic energy. The kinetic energy and interal energy of the other fragment are then easily calculated by the laws of conservation of energy and momentum. In this way we obtain not only a complete picture of the distribution of energy. We also obtain the probability distribution for each dissociation channel. The anisotropy provides information on the mutual orientation of the velocity vector and the transition dipole moment, as well as the time scale of the dissociation.



Results have been published for NO2 [1], N-nitrosopyrrolidin (NNPy) [2], and t-butylthionitrite [3]. After excitation to the second absorption band of NNPy dissociation into two different electronic states of the NPy radical has been observed. In all three compounds the dissociation occurs on a purely repulsive potential energy surface and on a time scale of a few femtoseconds.

3D-REMPI: Three dimensional resonance-enhanced multiphoton ionization spectroscopy.

A new method recently developed in our laboratory is 3D-REMPI. The same apparatus as for VMI is used. However, the ionization laser is tuned over all REMPI resonances of NO, and for each ion that hits the detector the position and the wavelength are recorded. The figure below shows a histogram of the ion counts as a function of wavelength (horizontal axis) and distance from the center (vertical axis)of the detector. Blue and green colors indicates low ion count, yellow and red colors corresponds to large counts. The picture consist of many stripes that are narrow along the wavelength axis and extend along the R-axis. Each of these stripes corresponds to one REMPI transition, and hence to one particular quantum state of the NO fragment. A simultaneous fit to these data yields, for each quantum state, the velocity distribution, but also the population in this particular fragment channel.



VMI: Velocity Map Imaging

Photodissociation with a linearly polarized laser produces a velocity distribution of the fragments which is rotationally symmetric around the polarization direction. The NO fragments are then ionized by a second laser via a 1+1 REMPI process: The first photon resonantly excites a particular quantum state (v,J) of NO to a well defined intermediate level in the first electronically excited 2S-state, the second photon then excites to the continuum of the positive ion. These ions are accelerated towards a two-dimensional detector (MCP, see figure below). The vector from the center of the MCP to the position of ion impact is proportional to the in-plane component of the fragment velocity. The "ion image" is thus the projection of the velocity distribution onto a plane. The original three-dimensional velocity distribution can be reconstructed from the ion image by Abel inversion, or a fit of a model function.



References:
  1. Kessler, A.; Kensy, U.; Dick, B. NO product yield excitation spectrum of the S0 -> S2 transition of nitrosobenzene in a supersonic jet. Chem. Phys. Letters 289, 516-520, 1998.
  2. Reinhold Seiler, Uwe Kensy and Bernhard Dick, Fluorescence excitation and UV-UV double-resonance spectroscopy of the S0!S1(Lb) transition of 1,6-methano[10]annulene cooled in a supersonic jet, Phys. Chem. Chem. Phys. 3, 5373-5382, 2001
  3. Kessler, A.; Seiler, R.; Slenczka, A.; Dick, B. The UV-photodissociation of jet-cooled nitrosobenzene studied by fluorescence excitation spectroscopy of the NO fragment. Phys. Chem. Chem. Phys. 3, 2819-2830, 2001.
  4. Seiler, R.; Dick, B. Alignment and velocity distribution of the NO fragments from the UV photo dissociation of jet-cooled nitrosobenzene studied by LIF and Doppler profile measurements. Chem. Phys. 288, 43-50, 2003.
  5. Obernhuber, Th.; Kensy, U.; Dick, B; Velocity-map ion-imaging of the NO fragment from the UV-photodissociation of nitrosobenzene. Phys. Chem. Chem. Phys. 5, 2799-2806, 2003.
  6. Schmaunz, A.; Kensy, U.; Slenczka, A.; Dick, B; Velocity resolved REMPI spectroscopy: a new approach to the study of photodissociation dynamics. Phys. Chem. Chem. Phys. 11, 7115-7119, 2009.
  7. Wenge, A. M; Kensy, U.; Dick, B; photodissociation dynamics of N-nitrosopyrrolidine from the first and second excited singlet states studied by velocity map imaging. Phys. Chem. Chem. Phys. 12, 4644-4655, 2010.
  8. Schmaunz, A; Kensy, U; Slenczka, A; Dick, B; Photolysis of tert-Butylthionitrite via Excitation to the S-1 and S-2 States Studied by 3d-REMPI Spectroscopy, J. Phys. Chem. A 114, 9948-9962, 2010.
  9. Andreas M. Wenge , Andreas Schmaunz , Uwe Kensy and Bernhard Dick, Photodissociation dynamics of tert-butylnitrite following excitation to the S1 and S2 states. A study by velocity-map ion-imaging and 3D-REMPI spectroscopy, Phys. Chem. Chem. Phys., 2012,14, 7076-7089

Other publications of B. Dick,


Webmaster: K.ZiereisWed, 24. Feb 2010
Address: Instutut für Physikalische und Theoretische Chemie, Universitätsstrasse 31, 93053 Regensburg, Germany
Lehrstuhl für Physikalische Chemie - Prof. Dr. Bernhard Dick
Tel: +49 941 943 4486, Fax: +49 941 943 4488, E-Mail: Bernhard.dick[at]chemie.uni-regensburg.de