Excited ν₃ vibrational state of the Ar-HCN and Kr-HCN dimers

Abstract

Rotational spectra have been observed for an excited vibrational state of the Ar/Kr-HCN dimers in a Balle-Flygare Fourier transform microwave spectrometer with a high temperature nozzle that can be heated to ∼1300 K. The B, D_J, and H_J rotational constants found for the excited parent Ar-HCN isotopic species are 1602.475(3) MHz, 164.2(6) kHz, and 309(5) Hz, and for Kr-HCN they are 1176.4493(3) MHz, 41.7(1) kHz, and 56(1) Hz. The 14N quadrupole interaction χ_a(J=1) observed in the excited state is −2.824 (−3.239) MHz for Ar(Kr)-HCN, slightly smaller than the −2.844 (−3.268) MHz in the ground state. A substitution analysis based on the C and N isotopic species reveals that the C-N distance rs (CN) increases by ∼0.013 (0.020 Å) on excitation of Ar(Kr)-HCN. A similar analysis for free DC¹⁵N gives an increase in r_s(CN) upon ν₃ excitation of 0.0022 Å. Assignment of the excited state in the dimer as the ν₃C-N stretch of the HCN is proposed and discussed.