Predicting an ultraviolet-tetraherz double resonance spectrum of formaldehyde

Abstract

In preparation for performing a triple resonance experiment to study the Rydberg states of calcium monofluoride (CaF), a double resonance spectrum of formaldehyde will be recorded. A dye laser will populate a level in formaldehyde's first electronically excited state, and pure rotational transitions will be induced by applying a terahertz electric field. A terahertz spectrometer has been built for this purpose, and the principles of terahertz spectroscopy are described. The 4'0 vibronically allowed transition of the A1 A2 <-- X1A1 electronic transition was chosen for study. The dye laser will be tuned to 28307.13 cm'l (353.2679 nm) within this band in order to transfer population from the ... level in the ground state to the ... level in the excited state, according to b-type selection rules for electronic transitions. A Boltzmann distribution was used to determine that ... was the most populated state, and 50% of the molecules from this level will be transferred to the excited state. The new population differences created after electronic excitation will allow four rotational lines ... in the ground state, and ...in the excited state) to experience a significant gain in absorption coefficient compared to all other rotational transitions occurring in the ground state. These new absorption coefficients are calculated and compared against those for the ground state spectrum without electronic excitation, showing about a factor of 10 increase. The changes in the THz electric field as it propagates through the sample of formaldehyde are also described.