Optical–Optical Double-Resonance Absorption Spectroscopy of Molecules with Kilohertz Accuracy

Abstract

Selective pumping and probing of highly excited states of molecules are essential in various studies but are also challenging because of high density of states, weak transition moments, and lack of precise spectroscopy data. We develop a comb-locked cavityassisted double-resonance spectroscopy (COCA-DR) method for precision measurements using low-power continuous-wave lasers. A high-finesse cavity locked with an optical frequency comb is used to enhance both the pumping power and the probing sensitivity. As a demonstration, Doppler-free stepwise two-photon absorption spectra of CO 2 were recorded by using two milliwatt diode lasers (1.60 and 1.67 μm), and the rotation energies in a highly excited state (CO-stretching quanta = 8) were determined with an unprecedented accuracy of a few kilohertz.

Sub-Doppler Double-Resonance Spectroscopy of Methane Using a Frequency Comb Probe

“…The signal-to-noise ratio of the strongest ladder-type probe transitions is currently limited to around ten by the short interaction length. The absorption sensitivity can be increased by implementing an enhancement cavity for the comb probe, as was recently demonstrated in continuous wave optical-optical DR spectroscopy [32]. These improvements will allow detection of a larger number of weaker probe transitions with higher frequency accuracy and better precision on the line shape, thus confirming the assignment for higher J states.…”

What It Takes to Compute Highly Accurate Rovibrational Line Lists for Use in Astrochemistry

“…Figure a demonstrates how an SO 2 EH model deteriorates exponentially at high K a Figure b plots the δ Calc‑Expt of 12 C 16 O 2 60025 band at ∼12 500 cm –1 , Ames versus CDSD2019 …”

“…(b) Prediction error δ­(Calc – Expt) for 12 C 16 O 2 60025 band at ∼12 500 cm –1 , with Expt. data from ref .…”

Cavity Ring-Down Spectroscopy: Recent Technological Advancements, Techniques, and Applications

“…The process of two-photon absorption (TPA) involves the simultaneous intake of two photons of the same or different wavelengths to undergo a transition from one state to another, usually from a lower to a higher energy state. It is a nonlinear process which manifests at high optical intensities and allows the interrogation of transitions which are forbidden by one photon spectroscopy . Moreover, with TPA, Doppler-free spectral lines can be probed using two oppositely directed beams of same frequency photons which cancels the effect of the velocity of the sample molecules. , This has opened the doors for precision spectroscopy which can be used to verify and remodel the theoretical predictions of spectral parameters and for better understanding of the fundamental constants.…”

“…However, CRDS enables the buildup of intensity inside an optical cavity which can allow the TPA process even with lower optical powers. Thus, few works have emerged combining TPA with the CRDS technique for precision spectroscopy. ,− Recently, two diode lasers with wavelengths ∼1.60 and 1.67 μm were used in the near-IR region to determine the TPA of CO 2 in a ro-vibrational transition (60025)-(00001), which had a jump of 8 quantum numbers in the stretching modes and hence was not possible with a one-photon transition. The two sources were tuned close enough to keep the measurement Doppler-free through counter-directed beams, and their frequencies were evaluated precisely (with ∼3.2 kHz uncertainty) by measuring their beat frequencies with respect to an optical frequency comb (OFC).…”