Methane Isotope Studies with Cavity Ring-Down Spectroscopy

Cavity ring-down spectroscopy (CRDS), as a highly sensitive optical spectroscopic method, has been widely used during the past three decades. This idea is to use cavity decay time to extract the information of single pass transmission or reflectivity of the optical resonator. Because of its immunity to intensity fluctuation, high sensitivity and high repetition rate, CRDS have been practiced in various field including trace gas detection, radical studies, chemical reaction kinetics, combustion research, photo dissociation dynamics, atmospheric and environmental studies, deposited nanoparticles, etc.
Distributed feedback (DFB) semiconductor laser diodes are used in our CRDS setup. There are two different ways to tune DFB lasers - temperature and current tuning. A new design of temperature controller with more evenly wavenumber stepsize is introduced. Also, a feedback loop from absorption peak is used for stabilizing laser frequency. When using the optimum modulation amplitude as well as appropriate PID parameters, the laser jitter decreases by over 20 times.
Three DFB near-IR lasers are used to measure the line strengths of CH3D ro-vibrational transitions in the wavenumber regions of 6017.5 – 6031.5 cm^-­1 and 6046.5 – 6070.0 cm­^-1 using continuous-wave (cw) CRDS. The N2, O2 and CO2 pressure-broadening and pressure-shift coefficients of CH3D are also measured.
As one of the most important hydrocarbon prototype molecules, CH3D’s overtone band are analyzed using different methods including combination differences, temperature dependence and double resonance. The fundamentals of theoretical spectrum analysis, including group theory, rotational spectra of symmetric top molecules, vibrational transition types, Herzberg's character table, irreducible representation, and selection rules are also covered.
At last, we describe and test a novel cell for CW-CRDS. The cell is monolithic and maintains a rigid alignment of the two cavity mirrors. Two high-resolution and high-force PZTs are used to sweep the length of the cell by elastic deformation of the 2.86 cm outer diameter stainless steel tube that makes up the body of the cell. The performance of the cell is demonstrated by using it to detect the absorption spectrum of methane (CH4) at the wavenumber regions of around 6051.8 – 6057.7 cm­^-1.