J . Phys. Chem. 1994,98, 2328-2336
2328
Vibrational Relaxation and Radiative Rates of Ozone B. L. Upscbulte and B. D. Green' Physical Sciences Inc., Andover, Massachusetts 01810
W. A. M. Blumberg and S. J. Lipson Phillips Laboratory/Geophysics Directorate, Hanscom AFB, Massachusetts 01731 Received: July 13, 1993; In Final Form: November 29, 1993"
Collisional relaxation of the coupled v3, v1 stretching modes of 0 3 at 90 K has been investigated by monitoring time and pressure variations of the infrared emission at 9.6 pm. Spectral modeling was utilized to determine the temporal variations in the populations of each vibrational level. A single quantum collisional relaxation model was used to evaluate total decay rates out of each vibrational level in this highly coupled system. Total decay rates at several different pressures were subjected to a Stern-Volmer analysis and have yielded level specific rate coefficients for collisional quenching of 0 3 with up to three vibrational quanta. Quenching rate coefficients were found to be in the range of 10-14-10-15 cm3 molecule-' s-' for 0 2 , N2, and Ar collision partners. Vibrational Einstein coefficients for Og(v3) with up to three quanta were also determined. This represents the first experimental determination for the two- and three-quanta states. These Einstein coefficients compare favorably with detailed theoretical predictions.
Introduction The ozone molecule, 03,plays an important role in many photochemical processes, affects the thermal and radiative budgets of our planet, and contributes a significant fraction of the infrared emission of the upper atmosphere. Emission at 9.6 pm from vibrationally excited O3 in the v3 asymmetric stretch mode has been observed in the mesosphere below 100 km by rocket-borne circular variable filter spectrometers'-' and interferometers.5 These observations have occurred during quiescent day and night conditions and during a class I11 a ~ r o r a .Three ~ processes are known to contribute to the production of vibrationally excited ozone in the atmosphere.6~~These are (1) radiative absorption of planetary IR emission,
0 , + hv
-
03(v3)
(1)
(2) collisional energy excitation transfer,
and (3) three-body recombination of atomic and molecular oxygen.
0 + 0,+ M
-
+
03(v3) M
(3)
Laboratory measurements have*-" established that the recombination process significantly populates higher vibrational levels, u3 2 2, while processes 1 and 2 will excite4(v3= 1) predominantly. Comparisons between laboratory and field data of the characteristic 03 > 1 vibrational temperature, about 2000 K, have confirmed the importance of recombination in the upper atmosphere.4 This significant departure from local thermodynamic equilibrium makes modeling of mesopause signatures and local concentrations very difficult. Many infrared radiation models7J2J3 neglect the behavior of the higher vibrational levels, while more recent models utilize only rough kinetic rate estimates.4,6 Vibrational relaxation of O3and energy transfer between the vibrational modes have been measured previously14-zl and critically reviewed.22 Typically in these experiments, the u3 mode is excited with a pulsed CO2 laser. Changes in selected v 1 or v3 *Abstract published in Advance ACS Absrracts, February 1, 1994.
0022-3654/94/2098-2328$04.50/0
or combination mode populations are then monitored by either passive fluorescence14-16or a double-resonance technique. '7-21 The band centers of the O3 V I , VZ, and v3 fundamental modes are 1103,701, and 1042 cm-I, respectively. It is well established that the Coriolis interaction strongly couples the nearly resonant vl and v3 modes.19 In addition, collisional energy transfer between these states is very fast, on the order of lo6 s-I T o r r ' (3 X 10-11 cm3 molecule-' s-l at 300 K). Energy transfer to the v2 mode is much slower, order of lo3 s-I Torr', and deexcitation of the v2 mode to translation is slowest of all. Thus, ozone can be viewed as a system with two modes, (VI, v3) and YZ. Henceforth, we will refer to the ( V I , ~ 3 coupled ) modes 33 because emission from the v3 transitions dominates the observed spectrum. We reserve the symbol B to represent the number of vibrational quanta in the polyad states formed out of the ( V I , ~ 3 )coupled modes. The energy levels and intramolecular energy flow among the lowlying O3states are depicted in Figure 1. Most of the measurements to date have concentrated on single-quantum excitation into 03(0,0,1) where a simple (PI,~ 3 ) - ~ transfer 2 treatment is adequate. Bounds for the atmospheric relaxation of higher levels were extracted from the SPIRE d a t a b a ~ e . Rate ~ coefficients were estimated to be (1-5) X em3 molecule-' s-I for v = 4-6. An investigation of thevibrational deactivation of the 33 levels, b = 1-3, for collision partners of atmospheric relevance, 0 2 and Nz, as well as Ar is reported here. In the next sections we will describe the experimental techniques for production of excited 03containing up to five quanta; the spectral analysis of overlapped infrared vibrational emission; the kinetic analysis of the cascade feed into each level and the collisional and radiative relaxation out of each level; and finally the Stern-Volmer analysis which yields both collisionalrelaxation ratecoefficients for the 53 coupled polyad states and the Einstein coefficientsfor v3 states containing up to three quanta. Experimental Section These experiments were performed using the cryogenic LABCEDE facility at the Air Force Phillips Laboratory. This facility, shown schematically in Figure 2, is composed of a cylindrical vacuum chamber, 3.4 m long and 1 m in diameter, and a thermal shroud cooled to 80 K with a slow, continuous flow of liquid nitrogen. The atmospheric gases of interest are cooled 0 1994 American Chemical Society
Vibrational Relaxation and Radiative Rates of Ozone
5000 -
om3
The Journal of Physical Chemistry, Vol. 98, No. 9, 1994 2329
+e Dyad Energy Exchanges (Fast)
2 Polyad Energy Relaxationvia v 2 States
'b3
20v3 3m3 Ozone Vibrational Mode
-
4m3
Figure 1. Ozone vibrational energy levels and schematic of energy flow in collisional processes. For each energy level graphed the upper number represents vibrational quanta in each mode; the number below the line represents the energy of the vibrational level in cm-l. Thermal Shroud
?-
i
lm
I Computer I Figure 2. Schematic of the cryogenic LABCEDE Facility.
in a heat exchanger attached to the thermal shroud and then enter the reaction chamber at one end through a large porous tube array. These gases traverse the longitudinal axis of the chamber under essentiallyplug-flow conditions and are exhausted through a 3 2 4 . diffusion pump backed by a Roots blower/fore pump combination (effective pumping speed of 2.6 X lo4L s-l). The electron beam propagates transverse to the longitudinal axis of the vacuum chamber, approximately 1 m from the upstream end of the tank. Two coils, positioned coaxial with the electron beam, provide a magnetic field of up to 70 G at the center of the reaction chamber that confines the secondary electrons. Vibrationally excited ozone was produced in the reaction chamber by two different methods. In the first method, chemical processes initiated by electron bombardment of 02-containing gas mixtures. Significant 0 3 ( % 3 ) emission at 9.6 pm was observed in electron-irradiated mixtures of between 5 and 40% of 0 2 in both Ar and N2. Three-body recombination at the low pressures (
