Cyanoalkynes and Cyanopolyynes: From Crossed Beam Experiments to Astrochemistry - Polyynes: Synthesis, Properties, and Applications - page 283

FIGURE 14.1 Schematic top view of the crossed molecular beam apparatus. The two
pulsed beam source chambers and the detector (electron impact þ quadrupole mass
filter) rotating chamber are visible. In the case of the CN radical beam source, the
carbon rod holder and the incident laser beam are also sketched. The chopper wheel
and the cold shield are also shown.

laser irradiation. The rotational direction of the carbon rod is reversed by
changing the polarities of the motor via a voltage switcher box. This has
revealed to be essential for an homogeneous consumption of the carbon rod
which guarantees the long term stability and reproducibility of the source.
To produce a suitable CN(X 2 þ ) beam, the 266 nm and 30Hz output of a

Spectra Physics GCR-270-30 Nd:YAG laser was focused with 30mJ per
pulse on the carbon rod to a spot less than 0.4mm in diameter. The super-
sonic expansion relaxes the diatomic radicals in their ground vibrational
level and lowest rotational levels. When more than 30mJ per pulse were
used, some vibrationally excited states of CN radicals were seen to survive
the supersonic expansion [85]. The laser beam entrance channel is comple-
tely isolated from the second source region to avoid reaction of CN radicals
in the first source with background reactant molecules. The pulsed valve
operates at 60Hz, 80ms pulses, and 4 atm backing pressure of neat nitrogen.
The laser-ablated species react with the nitrogen in the laser-ablation zone,
and, hence, form the cyano radicals in situ. The nitrogen reactant acts as a
seeding gas as well. The mechanism of formation of CN radicals has not
been well established. The possible reactions between molecular nitrogen
and the ablated atomic carbon and C 2 ,

3 P j Þþ

X 1 g Þ! X 2 þ Þþ

4 S

C ð N 2 ð CN ð N ð Þ , ð 14 : 3 Þ

X 1 g Þþ X 1 g Þ! X 2 þ Þ C 2 ð N 2 ð 2CN ð , ð 14 : 4 Þ