Cavity Ring-Down (CRD) Spectroscopy

Reactive chemical intermediates, primarily free radicals, are of crucial importance to combustion, atmospheric chemistry, and astrochemistry. In our High-Resolution Laser Spectroscopy Lab, we use the cavity ring-down (CRD) spectroscopy technique to study the structure and dynamics of free radicals, especially those with zero or low fluorescence quantum yield and, hence, cannot be detected by laser-induced fluorescence spectroscopy.

CRD is a highly sensitive laser absorption spectroscopy technique that can measure absolute absorption cross sections and has found wide applications in molecular spectroscopy. In CRD spectroscopy, exponential decay of laser intensity leaking from a high-finesse optical cavity is measured (Figure 1). The time constants of such decay when the cavity is empty and when it is filled with absorbing species are used to determine the absorption coefficient of the absorber. The absorption coefficient can be determined as:

where c is the speed of light, τ₀ is the empty-cavity ring-down time, and τ is the ring-down time for the cavity containing the absorber. With a typical experimental setup (cavity length L=0.5 m, mirror reflectivity R=99.995%), the empty-cavity ring-down time is:

The effective path length is, therefore, τ₀c=10 km. Thanks to the long path length, a minimum detectable absorption coefficient of less than one part per million (ppm) per pass can be achieved. The extremely high sensitivity of the CRD technique makes it particularly suitable for investigations of reaction intermediates, usually of low concentration.

The second major advantage of CRD spectroscopy is the insensitivity to fluctuations in incident radiation intensity. Unlike in traditional multipass absorption spectroscopy, where the intensity of the transmission is used to determine the absorbance, in CRD spectroscopy, it is determined from the change of the decay constant when the absorbing sample is introduced into the cavity. The intensity of the incident beam, therefore, doesn’t directly affect the absorption cross-section measurement.

Furthermore, because most molecules have absorptions due to vibrational motions in the IR region and electronic transitions in the UV/visible region even when the excited state is radiationless or dissociative, absorption-based techniques such as CRD are more versatile than fluorescence-based or photoionization/photoelectron techniques.

Three CRD spectroscopy apparatuses have been built in our High-Resolution Laser Spectroscopy Lab for the detection and investigation of free radicals:

A. Room-Temperature CRD Apparatus using a Flow Cell

In this experiment, free radicals are produced at room temperature by laser photolysis, often followed by free-radical reactions. All chemical reactions occur in the ring-down cell and are monitored by delaying the CRD probe pulse from the photolysis pulse on the order of 100s μs or ms. To extract the absorption due to free radicals, the spectrum taken without photolysis, i.e., solely due to the precursors, is subtracted from that with photolysis.

B. Jet-Cooled CRD Apparatus using a Pinhole- or Slit-Jet Pulsed Discharge Nozzle

In the CRD experiment using a flow cell, radical spectra are usually recorded at near ambient temperature at a total pressure of a few hundred Torr. Many energy levels, therefore, have considerable populations according to the Boltzmann distribution, resulting in congested molecular transitions. In addition, the spectral linewidth is large due to Doppler and pressure broadenings. Consequently, the rotational and fine structure of room-temperature spectra, and even their vibrational structure in many cases, are poorly resolved or not resolved at all. Supersonic jet expansion can drastically cool down the translational, rotational, and vibrational temperatures, simplify molecular spectra, and reduce the spectral linewidth. A pulsed supersonic expansion has been combined with a ring-down cavity for the high-sensitivity and high-resolution spectroscopic investigation of free radicals. Either a pinhole or slit jet nozzle (Figure 4) is used.

C. Continuous-wave (CW) CRD Spectroscopy Apparatus

Both the signal-to-noise ratio and resolution of CRD spectra have been significantly improved with the use of continuous-wave (CW) lasers. In Figure 6a, we illustrate a proof-of-principle spectrum of the (1, 0) band of the b¹Σ⁺_g-X³Σ^-_g "forbidden" transition of ambient oxygen in comparison with simulation.

Combining the Doppler-free saturated absorption spectroscopy and the CW CRD setups, our group is developing a novel cavity-enhanced two-photon spectroscopy technique.

Laser Sources

Output from a tunable pulsed dye laser pumped by a Q-switched Nd:YAG laser and its frequency doubling can be employed for CRD measurements in the visible and the UV regions (200-900 nm). The near-IR region (900-1500 nm) can be covered using H₂ Raman shift cells (single-pass or multi-pass). A difference-frequency mixer (DFM) that combines the outputs from the dye laser and the fundamental output (1064 nm) of the Nd:YAG laser extends the wavelength range to mid-IR (1.8 to 4.2 um). In the room-temperature CRD experiment, either an excimer laser or the third (355 nm) or fourth (266 nm) harmonics of a Nd:YAG laser is used for photolysis.

Production of Free Radicals

In our room-temperature CRD spectroscopy investigations, free radicals are usually produced by UV laser photolysis of judiciously selected precursors, often followed by free-radical reactions. Generating free radicals under jet-cooled conditions is more challenging because producing free radicals by photolysis is not applicable in this case: The pulse duration of the photolysis lasers, either Nd:YAG lasers or excimer lasers, is about 10 ns, while the photolysis beam needs to be focused along the downstream direction to a couple of millimeters. Therefore, the transit time, viz, the time for the produced free radicals to interact with the CRD laser, is limited to a couple of microseconds, much lower than the ring-down time. In a supersonic jet expansion, free radicals are usually generated using discharge or pyrolysis.

Experimental Details

• Room-temperature cavity ring-down spectroscopy (CRDS) apparatus using a gas cell. Output from a YAG-pumped dye laser is focused into a H₂ cell to generate near-IR (NIR, 900-1500 nm) radiation using stimulated Raman shifting (SRS). The desired ner-IR (NIR) second Stokes radiation is isolated using a long-pass filter or separated using a Perlin-Broca and coupled into a gas flow cell with ring-down mirrors mounted on both ends. Once the NIR light exits the cell, it is focused onto and detected by an amplified photodiode. We can also generate mid-infrared (MIR, 1.8-4.2 µm) radiation by difference-frequency mixing (DFM) of the fundamental (1064 nm) output of a seeded YAG laser and the output of the dye laser. We have high-reflectivity mirrors (R>99.98%) for nearly continuous coverage of visible, NIR, and MIR regions.

• Jet-cooled CRDS system. Free radicals are generated in a slit-jet or pinhole expansion using either discharge or laser ablation of a moving metal rod in the presence of organic precursors. The same laser systems as above can be used for recording vibronic spectra or survey scans. For high-resolution spectroscopy measurement, a CW Ti:Sapphire ring laser with tuning range 730-930 nm (13,700-10,750 cm^-1) is used to seed a YAG-pumped Ti:Sapphire amplifier. The Ti:Sapphire amplifier yields output pulse energies of 50-100 mJ with a linewidth ≤50 MHz through most of the tuning range of the ring laser. We use a multi-pass H₂ Raman cell to produce 3-8 mJ/pulse in the 9000-6500 cm^-1 range with a bandwidth of ~250 MHz. Alternatively, we can use DFM of the fundamental or second harmonics output of a seeded YAG and the Ti:Saphire amplifier output to produce narrow-band radiation from ≈2000 to ≈10000 cm^-1.