Observation and analysis of the ν2 and ν3 fundamental bands of the D2H+ ion

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Abstract
The high‐resolution absorption spectrum of the D2H+ molecular ion in the 1800–2300 cm−1 region has been measured in a discharge through a mixture of H2 and D2 using a tunable infrared diode laser source and a cooled hollow‐cathode absorption cell. A total of 72 new lines of D2H+ have been observed, as well as five previously measured in ion‐beam experiments by Wing and Shy, and these have been assigned to specific rotational transitions of the ν2 and ν3 fundamental bands. Two different and complementary theoretical models are used to fit these data: one is an A‐reduced asymmetric rotor effective Hamiltonian including the Coriolis and higher‐order rotational interactions between ν2 and ν3; and the other is a supermatrix model in which the matrix of the untransformed Hamiltonian is set up and diagonalized directly, using a large vibration–rotation basis that diagonalizes the vibrational energy. The former approach is less expensive and provides a better fit, but because of the large number of parameters varied may be more tolerant of incorrect assignments, whereas the latter is more expensive but may be more reliable because a smaller number of parameters is varied, most of the vibrationally off‐diagonal parameters being constrained at values from ab initio calculations. These analyses have made it possible to assign 11 other lines observed by Wing and Shy, giving a total of 88 assignments. The observed band origins, ν2=1968.17 cm−1 and ν3=2078.42 cm−1, and rotational constants are in good agreement with recent ab initio calculations.