Free Radical Chemistry in Crossed Beams

The bimolecular collisions involving small free radicals is being studied
by Brian Strazisar and Cheng Lin, shown below.  The rotating
detector crossed molecular beams apparatus is on the left.  In this case,
the two molecular beams are fixed at 90o and the detector may be rotated
in the plane of the beams to measure product angular and velocity distributions.

(Click on images for more detail)


The  experimental apparatus employed in our work is shown above and below.
We have recently studied  the four atom chemical reaction OH + D2  -> HOD + D.
This reaction is the most important benchmark system for understanding chemical reactions
in which more than one product vibrational mode may be excited.  It is also one of the
isotopic variants of a primary step in the combustion of hydrogen, and is the prototype reaction
in which a hydroxyl radical abstracts a hydrogen atom from a hydrocarbon, forming water.

In this experiment, the velocity and angular distributions of the D atom products are measured
at high resolution.  From conservation of energy and momentum, this allows us
to determine the vibrational distribution of the HOD counterfragment as a function of
its scattering angle..

The OH radicals are prepared by photodissociation of nitric acid in hydrogen carrier gas
using a 193 nm excimer laser. These OH radicals are then collimated by a skimmer and cross
a beam of deuterium molecules at 90 degrees.  The Rydberg tagging method
[See Schneider, Science 269, 207 (1995) and J.Chem. Phys. 107, 6175 (1997)] is used
to excite the nascent D atom products at the collision zone to high-n Rydberg levels around n = 40.
This is accomplished using several temporally and spatially overlapped lasers.  The first laser is in the
VUV at 121 nm (generated by 4-wave resonant enhanced 4-wave mixing in krypton vapor using
212 and 845 nm lasers) and excites the D atom to the 2p state.  Another laser near 365 nm then excites
this short-lived excited state up to a Rydberg level which is metastable.  These atoms then drift through a field-
free region to the microchannel plate where they are field ionized and collected.  By measuring the time
of arrival of the D atoms at the detector (25 cm away) we determine their velocity and
hence translational energy distributions.  By rotating the detector assembly with respect
to the fixed beams, we measure the angular distributions.

It was found in our experiment that the HOD vibrational energy disposal is highly vibrationally mode specific,
with the HOD product formed primarily with the OD local mode vibrationally excited in v = 2.
There is only a small amount of HOD bending excitation and essentially no energy in the OH vibration.
Our results disagree with a number of theoretical calculations based on potential energy surfaces
calculated in the 1980's and 1990's,  but agree very closely to recent results using quantum scattering methods
using a new potential energy surface.