Spectroscopic Techniques

In the McCall lab, we use a variety of spectroscopic techniques. In addition to geometric multipass cells, such as White and Herriott cells, we make extensive use of Fabry-Perot cavities to greatly increase the effective path length of the laser through the analyte of interest.

Cavity Ringdown Spectroscopy (CRDS)

Cavity ringdown spectroscopy (CRDS) is a sensitive technique for measuring the absorbance of a substance. It was originally developed as a tool for determining the reflectivity of mirrors, and has since been widely employed in measuring weak spectroscopic transitions and concentrations of trace gases. Early work with CRDS was limited to the visible region of the spectrum because the high-reflectivity mirrors required by the technique were not available in other regions of the electromagnetic spectrum. Recently, though, high-reflectivity mirrors have become available in the mid-infrared area, enabling high-sensitivity and high-resolution studies of weak vibrational bands, as well as vibrational bands of molecular ions that can only be produced at very low concentrations.

CRDS requires a stable optical cavity, formed with two highly reflective (R > 99.98%) concave mirrors. In the continuous-wave (cw) version of the technique that we use, a cw laser is directed into the cavity, and the length of the cavity is dithered with a piezoelectric crystal. As the length of the cavity changes, the frequency of light that is resonant with the cavity also changes. When the cavity becomes resonant with the laser, light begins to enter the cavity. Its intensity builds up as it reflects back and forth, but also leaks out at a rate that depends on the losses of the mirrors, the intracavity light intensity, and the presence of any molecules that absorb light at that frequency.

The light leaking out of the cavity is directed onto a detector, and when the signal reaches a predetermined threshold value, the laser beam is diverted away from the cavity by using an acousto-optic modulator. The intracavity light decays exponentially (or, "rings down") with a characteristic time constant τ, which is related to the product of the absorption cross-section and number density (Nσ, or effective absorbance, α), the reflectivity of the mirrors (R), and the distance between the mirrors (L). When an absorbing species is present, the intracavity intensity decays more rapidly owing to the absorption, and τ decreases. After measuring τ for several ringdown events at one frequency, we then tune the laser to a new frequency, measure the new time constant, and repeat until we have covered the spectral region of interest.

Our CRDS system has a minimum detectable absorption coefficient of ~2x10^-9cm^-1. This is equivalent to being able to measure an absorbance of <5 x 10^-7, which is several orders of magnitude more sensitive than a typical laboratory UV/Vis or FTIR spectrometer. Part of the reason for this increased sensitivity is that because we measure a time constant and not an actual intensity, the measurement is immune to intensity fluctuations in the light source. This is unlike typical absorbance measurements, in which intensity fluctuations are usually the dominant source of uncertainty in the absorbance measurement. As a direct absorption technique, CRDS is generally applicable to any molecule, as it does not depends on any "special" molecular properties. Finally, because this technique is gas-phase, there is no need to account for solvent shifts in the measurements, meaning that our measurements directly probe structure and intramolecular dynamics, and can be easily used to search for molecules in space, or as benchmark data for improving computational chemistry methods.

Cavity-Enhanced Absorption Spectroscopy (CEAS)

An alternative spectroscopic method involves actively locking the cavity length to the laser wavelength so they are constantly in resonance with one another. An error signal is generated using the Pound-Drever-Hall method, in which and EOM is used to add sidebands to the laser, then the back-reflection off of the cavity is captured and demodulated at the sideband frequency. This error signal is sent through electronics that split it into its low- and high-frequency components. The slow feedback signal is sent to the cavity piezo to correct the cavity length at up to 100 Hz, while the fast feedback signal is sent to a double-pass AOM to correct the laser wavelength at up to 60 kHz. There is always some residual frequency noise in the laser-cavity lock, and this gets directly converted into intensity noise in the cavity leak-out. To mitigate the effect of this noise on the experimental signal, CEAS is generally used with some form of modulation, either in the cavity length, the laser frequency, or the absorption of the analyte within the cavity. In our lab, we combine CEAS with either velocity- or concentration-modulation of molecular ions.

Cavity-Enhanced Velocity Modulation Spectroscopy (CEVMS)

Velocity modulation spectroscopy has been used over the past few decades to observe dozens of molecular ions. When a voltage is applied to a positive column discharge cell, a discharge is struck in which the cations are formed move toward the cathode. This imparts a Doppler shift to their absorption profiles. When an AC voltage is applied across the cell, the velocity of the ions is modulated. By passing a laser through the plasma and demodulating the detected laser intensity with a lock-in amplifier set to the plasma modulation frequency, ionic absorption signals can be detected without interference of neutral molecules. Until recently, the state of the art of this technique has involved using a modified unidirectional multipass White cell to get up to ~8 passes.

We have improved upon this technique by combining it with CEAS. Due to the symmetric nature of optical cavities, the cavity effectively acts as a bidirectional multipass cell. By demodulating the detector signal at twice the modulation frequency (2f), the absorption signals from both the velocity-modulated ions and the concentration-modulated excited neutrals can be extracted. Because of the different modulation mechanisms, the lineshapes and phases of the two signals can be easily distinguished and isolated. Because the optical cavity creates a great buildup of laser power of counterpropogating beams, it also allows for sub-Doppler saturation spectroscopy through the observation of Lamb dips.

Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular Spectroscopy (NICE-OHMS)

NICE-OHMS is by far the most sensitive spectroscopic technique we have implemented. The noise-equivalent absorption of our system is ~4x10^-11 cm^-1. It combines the enhanced absorption path length of CEAS with the low noise level of heterodyne spectroscopy. Two sets of sidebands are added to the carrier frequency of the laser. One set is used for Pound-Drever-Hall locking of the carrier onto a cavity resonance, as in other forms of CEAS. The other set of sidebands is used for heterodyne detection. This second set of sidebands must be spaced at exactly an integer multiple of the cavity free spectral range, so they can be efficiently coupled into the cavity, along with the carrier (primary frequency of the laser).

The absorption signal is extracted by detecting the heterodyne beat between the carrier and the two sidebands. Because one of the sidebands has a positive amplitude and the other has a negative amplitude, when there is no absorbing species within the cavity, there is no beat signal. As the laser is scanned over an absorption profile, one sideband is absorbed more than the other, and the imbalance in sideband intensity gives a beat signal that can be demodulated with an RF mixer. As the laser is scanned over an absorption profile, first one sideband is absorbed, then the other is, leading to successive positive and negative signals, giving a lineshape that looks somewhat like a first derivative. NICE-OHMS can also be used to detect dispersion signals. When one sideband is shifted by a dispersion of some intracavity species, that sideband doesn't get coupled as efficiently into the cavity, so this also creates an imbalance in the two sidebands and leads to a beat signal.

The biggest challenge in implementing NICE-OHMS is the locking of the laser to an optical cavity. But because of the noise-immune property of the technique, the requirements for locking are much less stringent than for other cavity-enhanced techniques. Once that is achieved, it is fairly straightforward to add sidebands spaced at an integer multiple of the cavity free spectral range.