Gas, Liquid & Solid Phase Photophysics and Photochemistry Using VUV-UV Laser
The high temperature conditions can be created by photoirradiation. The highly vibrationally excited state, namely a hot molecule, which has an equivalent vibrational temperature of 2000-4000 K, would be produced after a rapid internal conversion from an electronically excited state. The hot molecules are known to be formed by the VUV-UV laser irradiation of gaseous molecules. However, studies of the hot molecule reactions using lasers were limited to a small variety of molecules. The molecules other than aromatic hydrocarbons and olefins have not been well studied. The large molecules such as aromatic hydrocarbons larger than naphthalene have also not been well studied because they did not have a sufficient vapor pressure at ambient temperature. We have reported the hot molecule chemistry of large molecules such as naphthalene and 2, 2-paracyclophane.The large molecules, which have a low vapor pressure at room temperature, could be used for the experiments by elevating the experimental temperature for efficient vaporization and for enough absorbance at the laser wavelength. In the case of 2,2-paracyclophane, the experiments were carried out at 453 K. In these cases, product formations by the two-photon process were observed.
A single photon absorption would not be sufficient to induce a chemical reaction of such large molecules, which have many vibrational modes. It would be necessary to accumulate energy by a successive second photon absorption to overcome the activation energy of the chemical reactions as well as to fulfill the reaction rates higher than the collisional relaxation rates. The equivalent vibrational temperature of hot naphthalene and hot 2,2-paracyclophane formed by a single photon absorption of an ArF excimer laser light are 2320 K and 1655 K, respectively. In these cases, it would be reasonable to conclude that the hot molecules, which were created by the first photon, effectively absorbed the second photon because the hot molecules have a strong absorption at the laser wavelength as same as the parent molecules. Multiphoton reactions of hot molecule were first found for toluene (N. Nakashima et.al.) and azulene (J. Troe et.al.) in 1988.
The new reaction pathway would be expected for many molecules even though they hardly react by photolysis (single photon absorption), if hot molecules absorbed the second photon. The equivalent vibrational temperature with an internal energy of two photons is high enough to induce the reactions in the electronically ground state. The most important difference from the thermal reactions at the same temperature is that the products can be cooled down by collisional relaxation for the case of hot molecule. Therefore, different products would be expected. We have found several examples of new reaction pathways in photo-inert molecules. For example, we have found the two-photon reaction of biphenylene, whose internal conversion yield is nearly unity. In this study, coumarin was chosen as the target molecule in order to develop a new area of photochemical reactions under the concept of "thermal reaction induced by multiphoton absorption by hot molecules".