Kim Kilyoung, Johnson Alan M, Powell Amber L, Mitchell Deborah G, Sevy Eric T
Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, USA.
J Chem Phys. 2014 Dec 21;141(23):234306. doi: 10.1063/1.4903252.
Collisional energy transfer between vibrational ground state CO2 and highly vibrationally excited monofluorobenzene (MFB) was studied using narrow bandwidth (0.0003 cm(-1)) IR diode laser absorption spectroscopy. Highly vibrationally excited MFB with E' = ∼41,000 cm(-1) was prepared by 248 nm UV excitation followed by rapid radiationless internal conversion to the electronic ground state (S1→S0*). The amount of vibrational energy transferred from hot MFB into rotations and translations of CO2 via collisions was measured by probing the scattered CO2 using the IR diode laser. The absolute state specific energy transfer rate constants and scattering probabilities for single collisions between hot MFB and CO2 were measured and used to determine the energy transfer probability distribution function, P(E,E'), in the large ΔE region. P(E,E') was then fit to a bi-exponential function and extrapolated to the low ΔE region. P(E,E') and the biexponential fit data were used to determine the partitioning between weak and strong collisions as well as investigate molecular properties responsible for large collisional energy transfer events. Fermi's Golden rule was used to model the shape of P(E,E') and identify which donor vibrational motions are primarily responsible for energy transfer. In general, the results suggest that low-frequency MFB vibrational modes are primarily responsible for strong collisions, and govern the shape and magnitude of P(E,E'). Where deviations from this general trend occur, vibrational modes with large negative anharmonicity constants are more efficient energy gateways than modes with similar frequency, while vibrational modes with large positive anharmonicity constants are less efficient at energy transfer than modes of similar frequency.
利用窄带宽（0.0003 cm⁻¹）红外二极管激光吸收光谱法研究了振动基态CO₂与高振动激发态单氟苯（MFB）之间的碰撞能量转移。通过248 nm紫外激发制备了具有E' = ∼41,000 cm⁻¹的高振动激发态MFB，随后通过快速无辐射内转换至电子基态（S₁→S₀*）。通过使用红外二极管激光探测散射的CO₂，测量了从热MFB通过碰撞转移到CO₂转动和平动中的振动能量的量。测量了热MFB与CO₂之间单次碰撞的绝对态特定能量转移速率常数和散射概率，并用于确定大ΔE区域中的能量转移概率分布函数P（E，E'）。然后将P（E，E'）拟合为双指数函数并外推至低ΔE区域。P（E，E'）和双指数拟合数据用于确定弱碰撞和强碰撞之间的分配，以及研究负责大碰撞能量转移事件的分子性质。使用费米黄金规则对P（E，E'）的形状进行建模，并确定哪些供体振动模式主要负责能量转移。一般来说，结果表明低频MFB振动模式主要负责强碰撞，并控制P（E，E'）的形状和大小。在出现偏离这一一般趋势的情况下，具有大负非谐常数的振动模式比具有相似频率的模式是更有效的能量通道，而具有大正非谐常数的振动模式在能量转移方面比相似频率的模式效率更低。
