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Experimental observation of quantum interferences in CO-H$_2$ rotational energy transfer at room temperature

Hamza Labiad, Alexandre Faure, Ian R. Sims

TL;DR

This work reports room-temperature, state-selected measurements of rotational energy-transfer rate coefficients for CO in the $v=2$ state due to H$_2$ collisions and directly compares them to accurate 4D close-coupling calculations. Using time-resolved IR–VUV double resonance, the authors observe quantum interferences predicted for CO–H$_2$, validating the anisotropic part of the PES and offering a robust benchmark for astrochemical modeling of CO emission in warm environments. The results reveal clear even and odd $\Delta j$ propensity rules at room temperature, differing markedly from CO–He, and demonstrate the importance of PES details in high-temperature collisional dynamics. Together with the experimental method and theoretical framework, the study provides a pathway to improved high-temperature RET coefficients essential for non-LTE astrophysical modeling and PES validation.

Abstract

Using time-resolved infrared-vacuum-ultraviolet double-resonance spectroscopy, experimental room temperature measurements of state-to-state rate coefficients for rotational energy transfer within the X $^1Σ^+(v=2)$ vibrational state of CO due to H$_2$ collisions have been compared to accurate 4-D close-coupling quantum calculations. Theoretically predicted quantum interferences in the CO-H$_2$ collisional system are experimentally observed for the first time at room temperature, and excellent agreement between theory and experiment is observed. These results provide a valuable benchmark for validating the anisotropic part of the potential energy surface, thereby supporting the theoretical modeling of CO emission in warm astrophysical environments such as photodissociation regions.

Experimental observation of quantum interferences in CO-H$_2$ rotational energy transfer at room temperature

TL;DR

This work reports room-temperature, state-selected measurements of rotational energy-transfer rate coefficients for CO in the state due to H collisions and directly compares them to accurate 4D close-coupling calculations. Using time-resolved IR–VUV double resonance, the authors observe quantum interferences predicted for CO–H, validating the anisotropic part of the PES and offering a robust benchmark for astrochemical modeling of CO emission in warm environments. The results reveal clear even and odd propensity rules at room temperature, differing markedly from CO–He, and demonstrate the importance of PES details in high-temperature collisional dynamics. Together with the experimental method and theoretical framework, the study provides a pathway to improved high-temperature RET coefficients essential for non-LTE astrophysical modeling and PES validation.

Abstract

Using time-resolved infrared-vacuum-ultraviolet double-resonance spectroscopy, experimental room temperature measurements of state-to-state rate coefficients for rotational energy transfer within the X vibrational state of CO due to H collisions have been compared to accurate 4-D close-coupling quantum calculations. Theoretically predicted quantum interferences in the CO-H collisional system are experimentally observed for the first time at room temperature, and excellent agreement between theory and experiment is observed. These results provide a valuable benchmark for validating the anisotropic part of the potential energy surface, thereby supporting the theoretical modeling of CO emission in warm astrophysical environments such as photodissociation regions.

Paper Structure

This paper contains 6 sections, 2 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Theory
  3. Experimental Method
  4. Experimental procedure and data analysis
  5. Results and discussion
  6. Summary and conclusion

Figures (5)

  • Figure 1: Schematic diagram showing an overview of the experimental set up.
  • Figure 2: Typical scaled LIF decays (blue and red) for $j_i$ = 1 and $j_i$ = 4, with $[\text{H}_2]$ densities of 1.6 x 10$^{16}$ and 2.4 x 10$^{16}$ molecule cm$^{-3}$, respectively, and their exponential fits (black) used to estimate total removal rate coefficients. The LIF signals are recorded by tuning the pulsed IR laser on the R(0) and R(3) line transitions for state-selective preparation of $j_i$ = 1 and 4 respectively, tuning the pulsed VUV laser on the prepared initial state $j_i$ using the Q(1) and R(4) line transitions for $j_i$ = 1 and 4 respectively and monitoring in time the fluorescence signal using a solar blind PMT.
  • Figure 3: Short- and long-time delay spectra for $j_i$ = 0 at 293 K (blue) with multipeak Voigt fitting (black).
  • Figure 4: Measured state-to-state rate coefficients (squares) and theoretical state-to-state rate coefficients (circles), calculated using the 4D $\langle V_{15} \rangle_{20}$ PES for $j_\mathrm{i}= 0, 1$ and $4 \rightarrow j_\mathrm{f}$ at 293 K. Error bars correspond to 2$\sigma$ statistical errors.
  • Figure 5: Theoretical (solid lines) and experimental (circles) rate coefficients for the rotational (de)excitation of CO($j_i=4\to j_f=0, 1, 2, 3$) by normal-H$_2$ as a function of kinetic temperature. Experimental data points between 5.5 K and 20 K are taken from labiad_absolute_2022.