Vibrational Excitation Through Ionization

While the idea of LF explained the H2 data quite well [28], we were surprised by the magnitude of the oscillations in our I2 data [16], as, unlike H2,12 is not vibrationally cold at room temperature - the conditions for our experiment. Generally, thermal motion is detrimental to observing coherent motion. Thus, we took a long time scale run to get a more accurate measurement of the frequency of the vibrations, shown in Fig. 1.5. These data also exhibit a vibrational revival, from which the anharmonicity of the potential well can be determined. Indeed, the vibrational frequency accurately matched that of the ground state. 

Simulations of LF in a model molecule similar to I2 confirm that LF can produce coherent vibrations in hot molecules and, in fact, the amplitude of the motion increases with increasing temperature. This is unique in the field of 

Another remarkable aspect of LF, seen in the simulations, is that it can actually vibrationally cool the molecules. In particular, this means that the average internuclear separation Ravg should decrease as a function of increasing intensity (Fig. 1.6). This is exactly what we observe experimentally in Fig. 1.7, even as the amplitude of the vibrations is increasing [29]. 

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The paper is organized as follows we first discuss vibrational excitation through various mechanisms, including ionization, / -dependent depletion, and bond-softening. We then present evidence for electronic excitation and consider multiphoton excitation, inner orbital ionization, and excitation through recollisions. Several applications of these interactions are presented, followed by our conclusions. 

The distortion caused by the field allows an electron to pass from the molecule to the tip if the applied potential is positive or from the tip to the molecule if the potential is negative. This is called field ionization (FI), and the electron transfer occurs through quantum tunneling. Little or no vibrational excitation occurs, and the ionization is described as mild or soft. 

In this relation, ao 10 cm is the gas-kinetic cross section, and / is an ionization potential. It gives numerical values of the cross section of about 10 cm for electron energies about 1 eV It is much lower than relevant experimental cross sectiorrs, which are about the same as atomic values (10 cm ). Also experimental cross sections are nonmonotonic functions of electron energy and the probabihty of mrrlti-qrrantrrm excitation is not very low. Schultz (1976) presented a detailed review of these cross sectiorrs, which indicates that vibrational excitation of a molecule AB from vibrational level wi to 2 is usually not a direct elastic process but a resonant process proceeding through formation of an intermediate non-stable negative ion ... 

Nitrogen molecules electronically excited through VE relaxation can produce atomic oxygen and ozone by reactions (6-72) and (6-73). The VE-relaxation processes are adiabatic, related to energy transfer from heavy particles to electrons, and therefore their probability is relatively low. The kinetics of these processes is similar to that of associative ionization of vibrationally excited molecules (2-34). Another mechanism of direct energy transfer from vibrational excitation to ozone production is related to disproportioning in the collision of vibrationally excited oxygen molecules ... 

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