Interdependence of guest radiationless transitions and localized phonon structure: NH and ND(A ³Π) in rare gas lattices

Time and wavelength resolved fluorescence techniques are used to study vibrational relation of NH and ND(A ³Π) substitutional guests in rare gas lattices. Temperature dependent localized phonon spectral structure is analyzed to show that the ground X ³∑^- state undergoes near free rotation, while the excited A ³Π state librates. The barriers to free rotation in A ³Π reflect the anisotropy of the attractive, polarizability dependent NH-lattice interaction. The vibrational relaxation energy gap law is strongly violated, with NH rates 10-10² higher than ND rates. These rates are independent of temperature for T ≤ 25 °K in Ar, and T ≤ 37 °K in Kr; however, intersystem crossing by A ³Π (ν′ = 0) in Kr shows a dramatic temperature dependence resulting from stimulated phonon emission. It is proposed that the localized near free rotation mode accepts energy during vibrational relaxation, while delocalized lattice phonon modes accept energy during intersystem crossing. The average accepting frequency in Kr is only ≈1/2 the host phonon maximum frequency, thus indicating that single accepting frequency theories are only semiquantitatively applicable.