Laser Studies of Rotational Energy Transfer

Chemical Laser Studies of Rotational Energy Transfer 

HF laser following excitation of the J = 3i state of the v = level with the Pi(4) transition of the pump laser exhibited a double exponential decay. An initial rapid decay was observed on a time scale associated with the rise times of populations in other rotational levels (determined in separate experiments). A subsequent decay with a time constant about two orders of magnitude slower was also observed. This decay is believed to monitor momentum transfer collisions which result in a spreading of the initially sharply peaked velocity distribution into the thermal Gaussian distribution of velocities. The initial rotational relaxation corresponded to a rate constant greater than 2.2 X 10 cm molecule s the secondary momentum transfer rate constant was observed to be 3.1 0.6 x 10 cm molecule s .  

In order to determine the relative rate constants for coupling of rotational energy to nearby rotational states, a careful series of experiments were performed in which the temporal variations in absorption of the cw laser was observed when the laser was tuned for absorption by levels differing by 1 to 4 rotational quanta from the excited level. Table 3.3 lists the results of these studies. 

Examination was made of a kinetic model for rotational relaxation originally employed by Polanyi and Woodall in their treatment of R- T relaxation processes. That is, the rate constant for a molecule in p = 1, / = / for transfer to the p = I, J = j state in a collision with a ground-state partner which goes from v = 0, J=k to v = 0, J=l was represented with an exponential form based on the rotational energy discrepancy for the collision  

On this basis it was possible to calculate the fraction of molecules in a given J level that relax by rotational collisions with AJ = 1, 2, 3, etc. It was found that the losses from J = 3 occurred as 42.5% by A/ = 1 31.5% by A7 = 2 23.0% by A/ = 3 and 3% by A7 3. The importance of multiquantum rotational energy transfer processes is clearly demonstrated by these results. 

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