Cavity Ringdown

     By comparison with conventional absorption spectroscopy that involves the measurement of a small change in a large signal due to radiation transmitted through an absorbing material, causing a condition of a large background that results in a small sensitivity, cavity ringdown spectroscopy (CRD) is a sensitive method that relies on the measurement of the rate of decay of radiation trapped in a short cavity (e.g. our cavity length is 90 cm) formed between two highly reflective mirrors (~99.99 %), producing a long optical path (length ~9 km for a 90-cm cavity) that bestows great sensitivity. With CRD, one is thereby able to measure an extremely weak absorption with a large quantity or a minute quantity with strong absorption to achieve a detection limit otherwise unattainable with conventional absorption spectroscopy.

Recent Projects
     We have measured the vibration-rotational band 5-0 of CO in the electronic ground state for the first time, which showed the P branch to be stronger than the R branch. This result is unexpected on the basis of other measured overtone bands of CO that show opposite behavior.
We measured also a spectrum involving vibronic transitions,  and , of the CH3OO radical, produced from a reaction of O2 with CH3 radical from photolysis of propanone at 193 nm. The rotational parameters of the CH3OO in the state  were deduced.
We illustrate here the practice of cavity ringdown spectroscopy with a ‘photon bullet model.



  1. Detection of vibration-rotational band 5 – 0 of 12C16O X 1S+ with cavity ringdown absorption near 0.96 mm, C.-Y. Chung, J. F. Ogilvie, and Y.-P. Lee*, J. Phys. Chem. A 109, 7854 (2005).