| The theory for the production of photon echoes and the collision effects on the echo decay is developed and used to provide the method of measuring the total elastic collision cross section in gases. For the first time, it shows how the selection of time interval T between exciting pulses can distinguish between the small angle and large total cross sections.;(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI).;First a statistical theory is applied on the random velocity jumps under impacts. This yields.;(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI).;It is shown that the photon echo's amplitude is proportional to (VBAR)G(,2)(VBAR)('2). For plane wave exciting pulses, propagating in z direction, the velocity changing collisions effect is given by.;where (GAMMA) is the mean collision rate. Then consider the effect of elastic binary impact, the explicit form can be written as.;(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI).;where n is the density of the system, P(V) is the Maxwell-Boltzmann distribution function and (sigma)(V,(theta)) is the differential cross section of A - B impact.;It further shows that a classification of 1nG(,V) can be made on the variation of T, the time separation between exciting pulses, such that.;1nG(,V) (DBLTURN) (1/3)nK('2) T('3) (T < 5ns).;(DBLTURN) -nT (T > 1(mu)s).;(DBLTURN) -nT (T < 0.5(mu)s).;Where is the diffusion cross section, is the total collision cross section including both large angle and small angle, and is the total collision cross section by large angle only.;Assume a Van der Waals type intermolecular interactions, quantum mechanical consideration will yield a total elastic collision cross section = 2.4 b(,0)('2), where (b(,0))('5) = 3(pi)('C)/8(H/2PI)V and C is the Van der Waals interaction constant.;Measurements of the total elastic collision cross section of gases are carried out in echo relaxation experiments. Echoes are induced in SF(,6) stimulated by two pulses from two Q-switching CO(,2) laser on P(16) line.;Then the buffer gas is introduced sequentially in small amount of few millitorr, and echo intensity and the system pressure are recorded.;The relaxation of relative echo intensity is analyzed as a function of system pressure.;1n(I/I(,0)) = -4nT = -mP'.;The buffer gases studied include He, Ar, N(,2), O(,2), H(,2), HCl, CO(,2), SiF(,4), C(,2)H(,2), and CClF(,3). Experimental values of total collision cross section of these gases are as follows. In the unit of 10('-18)m('2), (,SF(,6)-He) = 1.0,;(,SF(,6)-O(,2)) = 2.1, (,SF(,6)-Ar) = 2.0, (,SF(,6)-H(,2)) = 1.2, (,SF(,6)-N(,2)) = 2.2,;(,SF(,6)-CO(,2)) = 2.3, (,SF(,6)-C(,2)H(,2)) = 2.2, (,SF(,6)-SiF(,4)) = 2.7, (,SF(,6)-CClF(,3)) = 3.1, and (,SF(,6)-Hcl) = 3.4.;A comparison is made between the calculated and the experimental values.;This study indicates that the photon echo technique is capable of measuring the total elastic collision cross section. However the experimental capability is constrained in that variation of T is limited and the observation of a transition from to can not be achieved. The measured data of are the total elastic collision cross sections. |
