| The helium nanodroplet isolation technique provides an unique opportunity to study elementary chemical reactions in the ultracold liquid helium environment. The main question addressed in this work is how the liquid helium environment affects chemical reaction dynamics. The discussion is limited to the case of unimolecular reactions.;First, it is shown how the spectrum of elementary excitation of helium droplets can be obtained. This spectrum is then used for an evaluation of microcanonical thermodynamic functions of helium droplets at low temperature, as well as for a description of the process known as evaporative cooling.;Second, Feynman's microscopic theory of liquid helium is extended to the case of helium droplets and other systems of liquid helium in confined geometries. Possible effects due to the finite size of helium droplets are also discussed. The extension of the microscopic theory allows for the prediction of the dynamic properties of helium droplets. The model of unimolecular reactions in conditions of collisional energy transfer is adapted for the case of unimolecular reactions in the liquid helium environment. The energy transfer rate function, in this case, is evaluated based on the dynamic properties of liquid helium and helium droplets.;Finally, the experiments on mass depletion spectroscopy of the NO 2 molecule in the regions below and above its gas phase dissociation threshold are described. In the first experiment, a mass depletion spectrum in the region 17700--18300 cm-1 is recorded. Gas phase NO 2 is believed to be vibronically chaotic at these energies. Transitions are broadened and blue-shifted relative to those of the gas phase by 7cm -1. Modest dispersion is consistent with quantum chaos in NO 2. It is shown that the relaxation is dominated by interactions of NO 2 with helium nearest neighbors.;In the second experiment, the guest-host dynamics of NO2 embedded in Hen droplets have been examined by recording depletion spectra of mass spectrometer signals at m/z values of 8 (He 2+), 30 (NO+), and 46 (NO2 +), throughout the wavelength range 340--402 nm. Above the gas-phase dissociation threshold (D0), it is shown that there is no net unimolecular decomposition all the way up to D 0+4300cm-1. To within the experimental uncertainty, it is found that reaction products do not leave the droplets. This is attributed to efficient relaxation and (at the highest energies examined) recombination within the droplets. |
