Accurate rest frequencies of submillimeter-wave lines of H2D+ and D2H+

We re-examined the submillimeter-wave transition frequencies of H2D+(J= 1 (10) - 1(11), at 372.4GHz) and D2H+(J= 1(10) - 1(01) at 691.7 GHz) to resolve suggested slight difference in velocity (V-LSR) of these species detected in the cold pre-stellar core 16293E recently. Both H2D+ and D2H+ were generated in a magnetically confined extended-negative glow discharge of a gaseous mixture of H-2/D-2/Ar. A combination of small improvements in various aspects of the measurements such as double modulation technique combined with a conventional frequency modulation and magnetic field modulation and more efficient signal accumulation method allowed us to improve signal-to-noise ratio, and thus to determine the transition frequencies more accurately. Both transition frequencies for the H2D+ and D2H+ lines have been thus determined to be 372421.385(10) and 691660.483(20) MHz, respectively. These precise rest frequencies suggest that the VLSR of H2D+ and D2H+ in the pre-stellar core 16293E are indeed different as indicated in a recent astronomical observation. In addition, in this investigation, another transition of H2D+ which falls in this frequency region, J = 3(21) - 3(22) transition, has been observed at 646430.293(50) MHz. As H2D+ is a lightest asymmetric-top molecule and it is difficult to predict the rotational transition frequencies by using the effective asymmetric rotor Hamiltonian, any new observation of the rotational lines will be useful to improve the molecular parameters. The molecular constants for the ground state have been obtained for H2D+ and D2H+ by fitting these new measured frequencies together with the combination differences. (c) 2005 Elsevier Inc. All rights reserved.