Nuclear quadrupole hyperfine quantum beats in HCl and pyrimidine measured by the pump-probe method using REMPI/LIF detection

Coherent time resolved IR-UV double resonance spectroscopy with REMPI and/or LIF detection has been used to measure the nuclear hyperfine structure of a diatomic and a polyatomic molecule. The pump-probe technique was applied and the experimental set up was optimized to achieve highest spectral resolution. Following excitation of the HCl fundamental vibrational transition by a nanosecond IR laser pulse, the nuclear quadrupole coupling constants were determined to be eQq = −69.51(22) MHz for H³⁵Cl and eQq = −54.40(16) MHz for H³⁷Cl in the J = 1 and J = 2 states of the υ = 1 level. Nuclear (Cl) spin-rotation interaction was shown to be active with the corresponding coupling constant being C _I = 0.068(10) MHz for H³⁵Cl and C _I = 0.049(8) MHz for H³⁷Cl. For pyrimidine a C-H stretch vibration ν ₁₃ was excited and the quadrupole tensor elements for the rovibronic states J _{K a} ,K _c = 1₁₀ and 1₀₁ of the υ ₁₃ = 1 level were found to be χ _aa = −3.095(10) MHz, χ _bb = 0.227(10) MHz and χ _cc = 3.322(10) MHz. In this case the residual frequency error was reduced to 8 kHz. The results of these jet experiments independently confirm those from millimeter wave and microwave measurements on static gas samples.