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 , N-nitrosopyrrolidin (NNPy) ,
and t-butylthionitrite . 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
3D-REMPI: Three dimensional resonance-enhanced multiphoton
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
- 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.
- Reinhold Seiler, Uwe Kensy and Bernhard Dick, Fluorescence excitation and UV-UV double-resonance spectroscopy of the S0!S1(Lb) transition of 1,6-methanoannulene cooled in a supersonic jet, Phys. Chem. Chem. Phys. 3, 5373-5382, 2001
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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