Simulation of the hyperfine-rotational spectrum of BrF due to electric and nuclear magnetic dipole transitions

The transition dipole of the hyperfine-rotation spectra of <em>J</em> = 1←0 within the vibronic ground (<em>X</em><sup>1</sup>Σ, <em>v</em> = 0) state of BrF are derived, and thus, the transition selection rules are summarized as: Δ<em>J =</em> ±1; Δ<em>F</em><sub>1</sub> = 0, ±1 and Δ<em>F</em>= 0, ±1, and those of Δ<em>F</em><sub>1</sub> = Δ<em>F</em> are intense while those of Δ<em>F</em><sub>1</sub> ≠ Δ<em>F</em> are weak. Some spectral lines are resulted from both the electric and nuclear magnetic dipole transitions due to perturbations, however, the magnetic only contributes about one-billionth in the spectral intensity. The spectral linewidth is determined to be about 18 kHz by calculating the spectral transition probability. The obtained spectral linewidth and relative intensities are consistent with the experimental results. Additionally, the hyperfine-rotation spectral positions are determined by diagonalizing the Hamiltonian matrix in the basis of |<em>JI</em><sub>1</sub><em>F</em><sub>1</sub><em>I</em><sub>2</sub><em>F</em>>, which is also in good agreement with the experiments within 10<sup>-8</sup> (one-fiftieth of the spectral line width). Hence, the microwave hyperfine-rotation spectrum is simulated. In addition, we find that, the nuclear spin-spin interaction not only slightly shifts the hyperfine-rotation spectral positions but also changes the sequence of the spectra. As to those constants unavailable molecules, the fairly precise molecular constants can be achieved by quantum chemical calculation, say for example employing MOLPRO program, and then the simulated spectra can guide the spectral assignment. Besides the guidance of spectral assignment, our results are helpful for other applications related as well, for example, absolute single quantum state preparation.