Mitchell Deborah G, Johnson Alan M, Johnson Jeremy A, Judd Kortney A, Kim Kilyoung, Mayhew Maurine, Powell Amber L, Sevy Eric T
Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, USA.
J Phys Chem A. 2008 Feb 14;112(6):1157-67. doi: 10.1021/jp0771365. Epub 2008 Jan 18.
Relaxation of highly vibrationally excited 1,2-, 1,3-, and 1,4-difluorobenzne (DFB) by collisions with carbon dioxide has been investigated using diode laser transient absorption spectroscopy. Vibrationally hot DFB (E' approximately 41,000 cm(-1)) was prepared by 248 nm excimer laser excitation followed by rapid radiationless relaxation to the ground electronic state. Collisions between hot DFB isomers and CO2 result in large amounts of rotational and translational energy transfer from the hot donors to the bath. The CO2 nascent rotational population distribution of the high-J (J = 58-80) tail of the 00(0)0 state was probed at short times following the excimer laser pulse to measure rate constants and probabilities for collisions populating these states. The amount of translational energy gained by CO2 during collisions was determined using Doppler spectroscopy to measure the width of the absorption line for each transition. The energy transfer probability distribution function, P(E,E'), for the large DeltaE tail was obtained by resorting the state-indexed energy transfer probabilities as a function of DeltaE. P(E,E') was fit to a biexponential function to determine the average energy transferred in a single DFB/CO2 collision and fit parameters describing the shape of P(E,E'). P(E,E') fit parameters for DFB/CO2 and the previously studied C6F6/CO2 system are compared to various donor molecular properties. A model based on Fermi's Golden Rule indicates that the shape of P(E,E') is primarily determined by the low-frequency out-of-plane donor vibrational modes. A fractional mode population analysis is performed, which suggests that for energy transfer from DFB and C6F6 to CO2 the two key donor vibrational modes from which energy leaks out of the donor into the bath are nu11 and nu16. These "gateway" modes are some of the same modes determined to be the most efficient energy transfer modes by quantum scattering studies of benzene/He collisions.
利用二极管激光瞬态吸收光谱法研究了高振动态激发的1,2 -、1,3 -和1,4 -二氟苯（DFB）与二氧化碳碰撞时的弛豫过程。通过248 nm准分子激光激发制备出振动态热的DFB（E'约为41,000 cm⁻¹），随后快速无辐射弛豫至基电子态。热的DFB异构体与CO₂之间的碰撞导致大量转动和平动能从热供体转移到浴中。在准分子激光脉冲后的短时间内探测00(0)0态高J（J = 58 - 80）尾部的CO₂新生转动布居分布，以测量碰撞填充这些态的速率常数和概率。利用多普勒光谱法测量每次跃迁吸收线的宽度，确定CO₂在碰撞过程中获得的平动能。通过将按态索引的能量转移概率作为ΔE的函数进行排序，得到大ΔE尾部的能量转移概率分布函数P(E,E')。将P(E,E')拟合为双指数函数，以确定单次DFB/CO₂碰撞中转移的平均能量以及描述P(E,E')形状的拟合参数。将DFB/CO₂和先前研究的C₆F₆/CO₂体系的P(E,E')拟合参数与各种供体分子性质进行比较。基于费米黄金规则的模型表明，P(E,E')的形状主要由低频面外供体振动模式决定。进行了分数模式布居分析，结果表明，对于从DFB和C₆F₆到CO₂的能量转移，能量从供体泄漏到浴中的两个关键供体振动模式是ν₁₁和ν₁₆。这些“通道”模式与通过苯/氦碰撞的量子散射研究确定为最有效的能量转移模式的一些模式相同。
