Lu-Yuan Hao

Cylindrical mirror multipass Lissajous system for laser photoacoustic spectroscopy

A simple optical multiple reflection system is developed with two cylindrical concave mirrors at an appropriate spacing. The two cylindrical mirrors have different focal lengths and their principal sections are orthogonal. The alternate focusing of the two cylindrical mirrors at different direction keep the reflecting spots small. The reflecting spots fall on Lissajous patterns on the cylindrical mirrors. The mathematics for this optical system is described and the calculated coordinates of beam spots are very close matches of the experimental observations. The cylindrical mirror optical system is easy to construct and align, with a suitable method for obtaining long optical paths and a large number of passes in small volumes. In a photoacoustic spectrometer the beam family enhance the effective power in the photoacoustic cell and thus the signal-to-noise ratio of photoacoustic signal. An experimental result for photoacoustic spectrum of HDSe gas is given.

A highly sensitive photoacoustic spectrometer for near infrared overtone

By optimizing the size of a photoacoustic cell for decreasing acoustic loss and noise, and using a proper multipass arrangement for increasing light power in the cell and optical coupling to the acoustic wave, a high-resolution external laser photoacoustic spectrometer with a detection sensitivity of 6.35×10−9 cm−1 (1:1 signal to noise level) has been developed. Using this apparatus, the high quality Doppler-limited overtone spectra of AsH3 (600) and H2Se (60) have been observed at room temperature in the wave number regions 11 500–11 650 and 12 600–12 925 cm−1, respectively. Results are presented which highlight the applicability of this apparatus to the spectroscopy of weak optical transitions.

A new cylindrical photoacoustic cell with improved performance

We report a new design for an acoustically resonant photoacoustic cell with high performance. A cylindrical cell with suitable size was selected so that the resonant frequency of the first radial mode was equal to that of a longitudinal higher mode. The ends of the cell have a pair of thin coaxial tubes which lengths are 1/2 and 1/4 of acoustic wavelength λ, respectively. The λ/4 acoustic resonance makes a coupling between the first radial resonance and the higher longitudinal resonance, then leads to an acoustic energy concentrating in the middle of the cell. Thus, the surface loss was decreased, the acoustic quality factor and pressure amplitude increased obviously as compared with conventional radial resonant cylindrical cell. The ability to measure absorption coefficients as low as 2.43×10−9 cm−1 (1:1 signal to noise level) has been achieved.

High resolution photoacoustic spectrum of AsH3 (600A1E) bands

The high resolution spectrum of arsine in the region of 11470-11650 cm(-1) was recorded by a sensitive laser photoacoustic apparatus. The vibration-rotation transitions of the local mode pair of bands (600A(1)/E) were assigned and their major vibration-rotation parameters were obtained by least-square fitting. The results indicate that the selection rules on the quantum number k have no effect in the high J values owing to strong Coriolis coupling. In addition, the intensities of vib-rotational transitions were estimated by comparison with standard water lines

Study of the stretching vibrational band intensities of XH4 molecules employing four-dimensional ab initio (X=C and Sn) and effective (X=C and Si) dipole moment surfaces

Stretching vibrational band intensities of XH4 molecules were investigated employing four-dimensional ab initio (X=C and Sn) and effective (X=C and Si) dipole moment surfaces (DMS) in combination with the local mode potential energy surfaces. The ab initio DMS of CH4 and SnH4 calculated at the coupled cluster CCSD(T) level of theory reproduced most of the observed intensities within a factor of 1.5. The effective DMS of CH4 and SiH4 were obtained by adjusting some selected high-order terms in the ab initio DMS to fit the observed intensities. They were applied to the corresponding deuterated isotopomers yielding better results than the ab initio DMS. The intensities of the combination bands are mainly due to the interbond cross terms in the DMS for SiH4, GeH4, and SnH4, while for CH4, both diagonal and cross terms are important. The relatively strong combination band that has comparable intensity with the pure overtone was predicted at the fourth local mode manifold for SnH4.

Absolute local mode vibrational band intensities of AsH3

The local mode vibrational absolute band intensities of AsH3 molecule were studied experimentally and theoretically. The stretching vibrational band intensities of AsH3 were derived from the infrared spectra recorded by a Bruker IFS 120 HR Fourier transform spectrometer. A three-dimensional As–H stretching dipole moment surface (DMS) was calculated by the density functional theory method. The DMS was used to calculate the absolute band intensities, which agreed well with the observed values. The inter-bond coupling terms in the DMS expansion are found to be essential in reproducing overtone and combination band intensities in the high-energy regions. 2002 Published by Elsevier Science B.V.