Large cross section for super energy transfer from hyperthermal atoms to ambient molecules

The experimentally measured cross section for super energy transfer collisions between a hyperthermal H atom and an ambient molecule is presented here. This measurement substantiates an emerging energy transfer mechanism with significant cross section, whereby a major fraction of atomic translational energy is converted into molecular vibrational energy through a transient collision-induced reactive complex. Specifically, using nanosecond time-resolved infrared emission spectroscopy, it is revealed that collisions between hyperthermal hydrogen atoms (with 59 kcal/mol of kinetic energy) and ambient ${\mathrm{SO}}_2$ result in the production of vibrationally highly excited ${\mathrm{SO}}_2$ with $>14000{\mathrm{cm}}^{-1}$ of internal energy. The lower limit of the cross section for this super energy transfer process is determined to be $0.53\pm 0.05$ Å${}^2$, i.e., 2% of all hard-sphere collisions. This cross section is orders of magnitude greater than that predicted by the exponential energy gap law, which is commonly used for describing collisional energy transfer through repulsive interactions.

Figure 1

Schematic PES of H+${\mathrm{SO}}_2$. Relative energies and configurations of each intermediate and transition state are adopted from theoretical calculations of Ballester et al. [20, 21].

Figure 2

Representative time-resolved emission spectra collected following 193 nm photolysis of a mixture of ${\mathrm{SO}}_2$ (40 mTorr), Ar (2 Torr), and HBr (200 mTorr). The spectral range of 1000–1500 ${\mathrm{cm}}^{-1}$, which contains emission from vibrationally excited ${\mathrm{SO}}_2$ and SO, is presented. Arrows indicate fundamental emission positions.

Figure 3

${\mathrm{SO}}_2$ vibrational energy distribution obtained from spectral fitting of the 1 $\mu \mathrm{s}$ spectrum. The distribution is characterized by an exponential decay at lower energy and a Gaussian function centered at higher energy.