Multipass observation

Direct spectroscopic measurements of absorptions could provide substantial and much-needed complimentary information on the properties of BLMs. Difficulties of spectroscopic techniques lie in the extreme thinness of the BLM absorbances of relatively few molecules need to be determined. We have overcome this difficulty by Intracavity Laser Absorption Spectroscopic (ICLAS) measurements. Absorbances in ICLAS are determined as intracavity optical losses (2JI). Sensitivity enhancements originate in the multipass, threshold and mode competition effects. Enhancement factor as high as 106 has be en reported for species whose absorbances are narrow compared to spectral profile of the laser ( 10). The enhancement factor for broad-band absorbers, used in our work, is much smaller. Thus, for BLM-incorporated chlorophyll-a, we observed an enhancement factor of 10 and reported sensitivities for absorbances in the order of lO- (24). 

Yakovlev and coworkers [402a, 402b] reported the IR absorption enhancement by mote than one order of magnitude for hydrocarbons adsorbed inside porous silicon, and assigned this effect to photon confinement in the microcavity acting like a multipass (Fabry-Perot type) cell. Recently, Jiang et al. [402c] observed a 50 times enhancement in the in situ IRRAS of CO adsorbed on Pd nanoparticles synthesized in cavities of Y-zeolite, as compared to the cases when the supports were ultrathin Pd films deposited directly on the zeolite or on amorphous alumosiUcate layer. 

Microwave measurements of uracil in a heated cell suggested the diketo form as the most abundant [54]. Brown et al. reported the first microwave measurements in a seeded molecular beam and also concluded that the diketo form was predominant [55]. Viant et al. reported the first rotationaUy resolved gas phase IR spectra of uracil [28]. This work employed a slit nozzle, an IR diode laser, and a multipass arrangement to obtain high resolution IR absorption spectra of the out-of-phase V(, C2—0, 4=0) stretching vibration. The rotational analysis unambiguously assigned the species to the diketo tautomer. Brown et al. also observed the diketo form of thymine in a seeded molecular beam, based on the hyperfine structure in the 14440-133,11 transition [56]. 

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