碰撞参量和碰撞能量对化学反应的影响

Impact Parameter and Collision Energy Dependence of Chemical Reactions(Ⅰ)

通过交叉分子束与高分辨、高稳定的激光光谱方法研究运动学限制的态 -态反应 :Ba( 1 S0 ) +HI( X1 Σ+ ,ν,J) Ba I( X2 Σ+ ,ν,J) +H( 2 S1 /2 ) ,并测得在 Ba与 HI的相对运动速度从 85 0到 1 30 0 m/s范围内产物 Ba I( ν=0 ,1 ,2 )振动态的转动分布 ,得到碰撞参量对平动速度的依赖关系 .如对 Ba I( ν=0 )振动态 ,碰撞参量极大值从 0 .4 60变为 0 .390 nm,碰撞参量的几率函数为截去头的高斯函数 ,较小的碰撞参量产生较高振动态和较低的转动态 .如相对运动速度为 94 0 m/s时 ,Ba I(ν=0 ,1 ,2 )的碰撞参量极大值分别为0 .4 60 ,0 .4 5 3和 0 .4 46nm.该态 -态反应截面随着碰撞能量的变化而变化 .当 Ba I与 HI的相对运动速度从85 0 m/s变化到 1 30 0 m/s时 ,反应截面先增大 ,达到极大值后逐步减小 ,在相对运动速度为 1 1 0 0 m/s时 ,反应截面有一极大值 .

Under the reaction of a heavy atom A with a heavy atom B\|light atom C molecule, information on the reactive impact parameters can be obtained. On the basis of previous work, we report the impact parameter and relative velocity dependence of chemical reactions. The state\|to\|state reaction, Ba(\{\}\+1\%S\%\-0)+HI(\%X\+1Σ\++, v, J\%)BaI(\%X\+2Σ\++, v, J\%)+H(\{\}\+2\%S\%\-\{1/2\}), was performed under crossed beam conditions using high resolution, high stable laser spectroscopy to determine vibrational and rotational resolved population distributions of BaI in relative velocity(\%v\-\%r) range of 850 m/s to 1 300 m/s. Velocity distributions of the reagents were measured using Doppler and Doppler\|free spectroscopy for Ba and time\|of\|flight method for the supersonic HI beam. The results, combined with previous experimental studies on this reaction system, show a remarkable collision energy dependence. Between low and high collision energies, a transition occurs in the intensity, width, and peak location of the product vibrational and rotational population distributions. The rotational distributions of the crossed\|beam experiments are extremely narrow but broaden at lower collision energies. As the collision energy is increased above 20.9 kJ/mol, the BaI rotational excitation is very near the energetic limit, and the maximum for the BaI(\%v\%=0) rotational population distribution moves from \%J\%=415.5 to \%J\%=538.5. In contrast, below the transition onset, the maximum remains unchanged around \%J\%=420.5. Morever, the peaks of the BaI(\%v\%=1) and BaI(\%v\%=2) rotational distributions appear at successively lower \%J\% values, as expected from energy conservation arguments. Detailed analysis of the collision energy dependence of the specific opacity functions offers insight into the role of conservation of energy and angular momentum in influencing this reaction. At low collision energies, the maximum reactive impact parameter, \%b\%\-\{max\}, is determined by an angular momentum(centrifugal) barrier. At collision energies larger than 20.9 kJ/mol, conservation of energy dictates the value of \%b\%\-\{max\}. These two processes are identified as the mechanisms that control the Ba+HI reaction cross section. The transition between the two mechanisms provides an interpretation for the bimodal character of the BaI product internal\|state distribution. When \%v\-\%r changes from 850 m/s to 1 300 m/s, the maximum impact parameter(\%b\%\-\{max\}) of BaI(\%v\%=0) changes from 0.460 nm to 0.390 nm. The shape of the derived specific opacity function \%P\-v(b\%)(truncated Gaussion function) changes with \%v\-\%r. The relative cross\|reaction appears to vary with reagent relative velocity \%v\-\%r, the cross\|section increases with \%v\-\%r and reaches a maximum at 1 100 m/s. Then, the cross\|section shows a more pronounced decrease at large collision energies. The behavior for low relative velocity indicates the existence of a reaction energy barrier.