High Resolution Cavity Ring-Down Spectroscopy of Radicals and Other Species

 

INTRODUCTION                     EXPERIMENT                   SPECTRUM                                       

Reactive chemical intermediates, such as free radicals, are of crucial importance for combustion and atmospheric chemistry. In recent years, our group has exploited the technique of cavity ring-down spectroscopy (CRDS) to observe the electronic spectra in the near infrared (NIR) of a number of organic radical intermediates, including particularly a number of organic peroxy radicals, RO2. However, those radical spectra have all been recorded at near ambient temperature at total pressures of a few hundred torr. Correspondingly rotational (and finer) structure has been poorly resolved or not at all. Obviously a great deal more could be learned from high resolution, rotational resolved, jet-cooled NIR CRDS spectra of RO2 radicals and other reactive intermediates. Such spectra would allow convincing correlaton of the spectra with the isomeric and conformeric structure of the organic radicals. They would also provide precise spectroscopic parameters such as rotational and spin-rotational constants, origin frequencies, etc., by which quantum chemistry computations could be benchmarked.

Since a number of open-shell species, e.g., RO2, have low-lying electronic state transitions in the NIR (1.0-2.5 mm or 10000-4000 cm-1 ) and all species have fundamental vibrational transitions in the mid IR (MIR, 2.5-20 mm or 4000-500 cm-1) it would be extremely desirable to extend jet-cooled studies from the visible into the NIR and the MIR regions. Moreover it would be useful to incorporate high resolution to improve both the information obtainable from the spectra and the instrument's sensitivity. In recent work, initial results are obtained from such a device. High spectral resolution is implemented by using a Fourier transform limited, Ti:Sa pulse amplified laser whose output is shifted to the NIR and MIR by stimulated Raman scattering (SRS) in H2. A pulsed, slit-jet discharge expansion has been located in a high quality ring-down cavity. The high sensitivity and high resolution of this instrument, as well as its intense, cold-radical production are demonstrated in the spectral results for H2O, O2, metastable A3Su+  N2, and the OH and CH3 radicals.

 

 

INTRODUCTION                     EXPERIMENT                    SPECTRUM                            

The experimental setup, consists of a high resolution, high energy, pulsed laser source, a Raman shifter, a pulsed slit-jet discharge expansion and an optical cavity. The laser source is based on a Ti:Sa pulse amplifier, which is seeded by a high resolution cw-Ti:Sa ring laser (Coherent 899-29, tunable from 730 nm to 930 nm). The pulse amplification is accomplished in Ti:Sa crystals pumped by a nanosecond Nd:YAG laser (Spectra Physics 170) operated at 20 Hz. This system can deliver 50-100 mJ pulse energies with a linewidth of around 40-60 MHz (FWHM). Radiation at a desired wavelength in the NIR and MIR is generated by SRS in a H2 cell. The bandwidth of output radiation is subjected to the pressure broadening in H2, resulting in a bandwidth in the IR of about 150 MHz.

The longitudinal mode spacing of our ring-down cavity is 224 MHz . Problems could arise from the fact that the mode spacing is comparable to the bandwidth of the radiation. However any problems of this nature were avoided by utilizing both longitudinal and transverse modes to form a near-continuum mode structure compared to the 150 MHz radiation bandwidth. To achieve this result, no mode matching optics were used to couple light into the cavity, and a lens (focal length 30 cm for NIR, and 3 cm for MIR) was used to focus all the transmitted light onto the InGaAs NIR detector or InSb MIR detector.
 

Our slit-jet with discharge expansion is an adaptation from earlier designs. The slit body and the solenoid mount, were made from aluminum. A 5.5 cm long aluminum poppet is actuated by two commercial General Valve solenoids with a commercial multi-channel driving circuit (General Valve Corporation, Iota one). A Viton sealing cord is trapped within the poppet. The slit on the front face of the slit body has the dimensions (L x W x H of 50 x 0.5 x 1 mm). The 300 mm width of the jet throat is defined by the blades jaw.

The discharge method was used to generate radicals in slit-jet expansion. The pulsed discharge voltage was provided by a home-made power supply with an output voltage up to 3 kV and a peak current up to 2.5 A. Typically, the radicals were probed at 10 mm downstream distance from the jet throat. The discharge voltage and the timing between probe beam, jet and discharge, was optimized to obtain a stable discharge and a maximum spectral signal. The optimized timing and discharge voltage varied from different buffer gas and precursor. Typically, the discharge is around 1 KV, and the trig of discharge should start 200--300 ms after the trig of jet valve due to the delay of mechanical motion, and then probe beam shots 100--200 ms later when the discharge current become stable.

  

INTRODUCTION                    EXPERIMENT                    SPECTRUM                                      

 H2O           O2           OH          N2(A3Su+)        CH3

Preliminary experiments have been performed to characterize the apparatus. Experiments were conducted in the NIR on  vibrational transition of H2O and the doubly forbidden transition of O2, respectively. For both of the two species, the transition intensities of the rotational lines are well known; therefore, they can be used as reliable thermometers in slit-jet expansion.

INTRODUCTION                    EXPERIMENT                    SPECTRUM                                  

 H2O           O2           OH          N2(A3Su+)        CH3

INTRODUCTION                    EXPERIMENT                    SPECTRUM                                      

 H2O           O2           OH          N2(A3Su+)        CH3

To test the capability of producing large densities of jet-cooled radicals, the spectra of n = 2 ← 0 vibrational band, Q(1.5) and Q(2.5) transitions of OH, were recorded using our apparatus.

 

INTRODUCTION                    EXPERIMENT                    SPECTRUM                    

 H2O           O2           OH          N2(A3Su+)         CH3

In addition to the jet-cooled radicals, the pulsed slit-jet discharge also generates jet-cooled ions and metastable species. An example of a CRDS spectrum of a metastable species is that of the B3Pg ← A3Su+ transition n'=2 ← n"=4 of N2. It was excited to the metastable A3Su+  state by discharging 2% mixture of N2 in argon at 1 atmosphere backing pressure. The entire n'=2 ← n"=4 vibronic band was recorded in the region of 7300 -- 7440 cm-1 with a noise level (standard deviation) of 0.2 ppm.


 

 

 

 

 

 

 

INTRODUCTION                    EXPERIMENT                    SPECTRUM                                     

 H2O           O2           OH          N2(A3Su+)         CH3

As a demonstration of the spectroscopic capability of the apparatus in MIR, the jet-cooled spectrum of n3 band of CH3 were recorded around 3.15 mm.