CHEMICAL LASERS FROM THE PHOTODISSOCIATION OF NITROSYL HALIDES

New NO laser emission from the photodissociation of BrNO has been observed between vib-rotational levels ofthe ground state of NO, using He and Br_ as buffer gases. Also, from the C1NO photodissociation several new NO vibro-rotational lines have been observed. The presence of a buffer gas was necessary to observe laser emission from BrNO and C1NO. The temporal behavior of the NO laser emission from the photodissociation of both nitrosyl halides has been studied.


INTRODUCTION
The photodissociation of NOX, specially NOC1 have been studied extensively during the last decades. Goodeve and Katz were the first to analyze the spectrum of NOC1 in the visible and ultraviolet regions.
Kistiakowsky 2 studying the photodissociation of NOC1 between 365 and 635 nm proposed two mechanisms to account for a quantum yield of nearly 2.
The first mechanism" NOC1 + hv NO + C1 (1.a) C1 + NOC1 NO + CI 2 (l.b) the second mechanism" NOC1 + hv NOCI* (2.a) NOCI* + NOC1 2NO + C12 (2.b) Although Kistiakowsky favor the second, Basco and Norrish 3, using the flashphotolysis technique, observed that the NO production during the photodissociation was independent of the pressure of inert gases added to the reaction, proving that the reaction followed the first mechanism and not the second.
Several authors 4-9 have corroborated the above mechanism and recently Reisler et al. 1 using vector correlations and ab initio calculations have studied in detail the photodissociation dynamics of NOC1 from the T1 (13A") and the S (IA").
A chemical laser is the coherent radiation collected from the population inversion obtained by breaking or reordering the bonds of one or several molecules.
To reach this situation is necessary to place our system as far away as possible from equilibrium. The states of the fragments involved can be electronic, vibrational or rotational. The techniques used to break or reorder the bonds of the molecules require inelastic collisions with photons, pumping by flash or other lasers, or electrons in discharges. The pure chemical lasers use only the exothermicity of the chemical reaction to produce the necessary population inversion. G. C. Pimental et al. [11][12][13] were the first to observe laser action following flash photodissociation processes.
Deutsch 5 using an electrically pulsed discharge observed more lines, up to v 11, flowing a mixture of NOC1 and He. Giuliano and Hess 6 obtained reversible laser emission from mixtures of C12 and NOC1 but did not analyze the V-R components.
We have observed several new (V-R) laser lines after flash-dissociating NOC1.
Using He and Br2 as buffer gases we have obtained for the first time NO laser emission from NOBr photodissociation.
During the rest of the article we summarized our own results.

EXPERIMENTAL
A concentrical suprasil lamp, filled with 20 Torr of Xe, 37 cm long, produced flashes that reached maximum intensity in 2 #s, discharging a 2.5/farad capacitor charged to 18 Kv. The cavity, 90cm long, was hemiconfocal and formed by a gold coated flat and a 2 meter of curvature spherical mirror. NOCI and NOBr were synthesized from C12, Br2 and NO and carefully purified from bulb to bulb distillations.
The lines were analyzed with a meter Jarrell Ash monochromator provided with a grating blazed at 5 microns. Gold doped infrared detectors were used at 77 K.
To avoid water atmospheric absorption the cavity was purged with dried nitrogen.

Observed Lines and Assignments
Tables and 2 show the measured wavelengths and the assignments of the NO laser emission from the photodissociation of NOCI and NOBr. As in most chemical lasers, due to their higher gain, the observed transitions have been assigned to the P branch. The NO laser emission reaches vibrational levels 9 to 8 in NOC1, but only 7 to 6 in NOBr. This behavior does not seem to be correlated to the strength of the corresponding halogen-nitrogen bond. The intensities of the observed lines are more evenly distributed in the NOBr than in the NOCI laser. It is remarkable to observe that a large part of the laser energy in NOC1 is concentrated in the 1-13/2 7-6 line.

Overlap with Atmospheric Water Absorption
The normal symmetric bending mode of water overlaps, around six microns, with the NO laser emission. To avoid this absorption we have purged the optical cavity   with dried nitrogen17'18. Several new lines were detected in the NOC1 laser and many other lines increased substantially their intensities. Figure 1 shows how the lines that increased most their intensities, when purging, sit on the maximum of the water band and those that decrease coincide with windows in the water spectrum. Figure 2 presents the effect ofpurging on total laser emission.

Buffer effects on Laser Intensities
The presence of a buffer gas was necessary to observe laser emission. Helium, chlorine and bromine were used as buffer gases. It seems that the lower vibrational bands only appear with He. When the halogens are used, reaction (1.b) becomes important and tends to populate the lower vibrational levels avoiding the population inversion in the final steps of the laser cascade.
As buffer pressure increases we have observed that the substate 2H1/2 becomes more intense while the 21-13/2 weakens 17. This behavior was attributed to a funnelling of population from 21-13/2 to 21-11/2. The higher degeneracies of rotational levels, for similar energies, can explain this population transfer.

Temporal Behavior
The time evolution of the laser lines has been studied 19. Figures 3 and 4(a), (b) show the temporal evolution of the rotational-vibrational transitions that correspond to the substates X21-I1/2 and X2II3/2 respectively. The start of the laser pulse defines  =0; it takes place -2.2/s after the beginning of the flash for NOC1 and _3.3/s for NOBr. This seems to indicate a lower gain for this laser. In general, the J shifting typical of rotational equilibrated chemical lasers is not observed. All transitions starting from the same level laser simultaneously, except the v 7 6 band. The relative intensities of the observed lines indicate that within a vibrational level the population among the rotational levels is rapidly distributed in both lasers. The NO formed from the dissociation of NOC1 appears mainly in v 7, being the P(8) transition of the substate l-la/2 the one that shows the maximum gain in both lasers. Within each vibrational band all the rotational transitions take place simultaneously. This behavior seems to indicate lack of rotational equilibration. However, the relative intensities and the pressures used in our experiments would seem to indicate that rotational equilibration has been achieved before lasing starts. Figures 5 and 6 show the time evolution of the laser cascade from CINO and BrNO, respectively. In both cases, the first band to appear corresponds to an intermediate vibrational level, originating two sequences. The lowest vibrational band is the last to appear in both lasers.