1. High resolution direct frequency comb spectroscopy of vinyl bromide and nitromethane in the CH stretch region
Bryan Changala1, Ben Spaun1, David Patterson2, and Jun Ye1
1JILA, NIST and University of Colorado, Boulder, CO
2Department of Physics, Harvard University, Cambridge, MA

2. Leveraging frequency combs for high resolution molecular spectroscopy
High resolution: width of comb modes ≈ 50 kHz
High bandwidth: cm-1
High throughput: simultaneous measurements
High sensitivity: effective path length ≈ m
Frequency
Cavity modes
Comb light
Kelvin

3. Outline Vinyl bromide: a “simple” case
Resolving spectral congestion
Accurate relative intensities in broadband spectra
2. Nitromethane: a progress report
Unhindered internal rotation
Complicated and subtle aspects of the asymmetric CH stretching + internal rotation dynamics

4. CH stretch modes of vinyl bromide
~ frequency elements, ~1 hr

5. Vinyl bromide band Near prolate top, only a-type observed
(though b-type transitions are allowed)
R(9)
79Br
81Br
R(0)

6. molecular constants High accuracy, high resolution spectra
Reliable relative intensities
Fast data acquisition times

7. Nitromethane, CH3NO2 E Free internal rotation 50 cm-1 22 cm-1 5.5 cm-1
V = 2 cm-1

8. Overview spectrum of CH3NO2 at 3.3 μm
In-plane
Out-of-plane

9. Model 4D CH stretch-torsion calculation
Asymmetric CH stretch modes are strongly coupled via torsion; vibration-vibrational angular momentum coupling

10. CH stretch motion with internal rotation
Consider the “out-of-plane” CH stretching motion…
Adiabatic internal rotation limit:
CH stretch fixed to ONO plane
Fast internal rotation limit:
CH stretch fixed to rotor

11. Model 4D CH stretch-torsion calculation
oop,
m = 0

12. Take a look at the out-of-plane CH stretch
In-plane
Out-of-plane

13. Out-of-plane CH stretch (m = 0)
0.2 cm-1

14. But wait, there’s more! rotation-torsion-vibration coupling
Total rotation
Vibrational angular momentum
Torsional
internal rotation

15. The in-plane CH stretch
Out-of-plane
In-plane CH stretch band appears to be highly perturbed—analysis ongoing!

16. Conclusions
Cavity-enhanced direct frequency comb spectroscopy + Buffer gas cooled molecule source = Large volumes of high resolution spectra
Continuing work: CH region of CH3NO2 exhibits non-trivial, highly coupled torsional-rotational-vibrational nuclear motion dynamics, a challenging experimental and theoretical problem!
Funding:
DARPA SCOUT, AFOSR, NIST, NSF-JILA PFC (JILA)
NSF GRFP (BC), NRC RAP (BS)

17. Buffer gas cooling frequency comb spectrometer

18. Buffer gas cooling of CH3NO2
Doppler width
Rotational temp.
Trot = 10.7(12) K