Difference Frequency Laser System Research Articles

Observation of the infrared spectrum of methyl cation CH+3

The vibration-rotation infrared spectrum of the ν3 fundamental band of the methyl cation CH+3 has been observed using a difference frequency laser system. The analysis of the spectrum demonstrates the D3h symmetry of the ion. This work marks the beginning of high resolution spectroscopy of simple polyatomic hydrocarbon cations.

Infrared studies of molecular ions. I. The ν3 band of NH4+

Infrared spectrum of the ν3 fundamental vibration-rotation band of the ammonium ion NH4+ has been observed. The ac discharge method recently developed by Saykally and his colleagues was used with a difference frequency laser system as the radiation source. Some 150 lines have been observed and assigned to P,q, and R branches up to J=10. An analysis of the low J transition gave the following molecular constants. ν0−2(Bζ)′ =3342.573 (21) cm−1, B′−(Bζ)′=5.523 53(70) cm−1, B′−B0=−0.049 997(19) cm−1, Zt=−7.59(80)×10−3 cm−1, Ft=−1.6(11)×10−4 cm−1, and Ds=7.2(32)×1p−5 cm−1. A calculated Coriolis constant was used to obtain the ν0 and b2d0 values as ν0=3343.130 (56) cm and b0=5.852 (31) cm−1. These results confirm the expected tetrahedral structure of this molecular ion and give an estimated equilibrium bond length of 1.026(5) Å. (AIP)

Reorientation and pressure broadening of IR or MW lines: New results in CH3F

A difference-frequency laser system has been used to measure the pressure-broadening parameters of both the Stark-split components and the degenerate line for the transition R(1, 1) in the parallel band of CH3F at 2866 cm−1. The analysis of the data shows that the individual Stark components are broader than the zero-field line. This result is shown to be in agreement with the predictions of the infinite order sudden approximation based on dominant dipole–dipole interaction, and can be interpreted in terms of interference effects between overlapping lines.

Observation of the Infrared Spectrum ofH3+

The infrared ${\ensuremath{\nu}}_{2}$ band of ${\mathrm{H}}_{3}^{+}$ has been observed. A direct infrared absorption method combining a liquid-nitrogen-cooled multiple-reflection discharge cell and a difference-frequency laser system has been used for the detection. Fifteen absorption lines have been measured in the region of 2950-2450 ${\mathrm{cm}}^{\ensuremath{-}1}$ and assigned. This is the first spectroscopic detection of this fundamental molecular ion in any spectral range.

Deconvolution of infrared spectra beyond the Doppler limit

It is shown that the deconvolution method of Van Cittert can be used reliably to enhance the effective spectral resolution by a factor of ~3 with data that exhibit a high SNR (~10(3)) and in which base line variations have been eliminated. Deconvolution of a Doppler-limited spectrum of C(6)H(6) measured on a difference-frequency laser system yielded linewidths of ~1.2 x 10(-3) cm(-1) (compared with the Doppler width of 3.6 x 10(-3) cm(-1) at 203 K). Extensive reliability tests of the deconvolution technique have been performed.

A study of the K = 3 nA1 - A2 centrifugal splitting in the 3 ν2 and 4 ν2 states of PH 3 using a difference-frequency laser system

A frequency tunable infrared source has been constructed by using the (Ar-laser) - (dyelaser) difference frequency method developed by Pine and applied to the observation of the overtone bands of PH 3 3 ν 2 ← 0 and 4 ν 2 ← ν 2 in the 3.4 μm region and 4 ν 2 ← 0 in the 1.6-μm region. A Stark modulation method was used to increase the sensitivity of detection. For transitions which were well modulated, the minimum detectable absorption coefficient was estimated to be ∼3 × 10 −7 cm −1 using a 3-m cell. Emphasis was placed on the observation of the A 1- A 2 splitting for K = 3 n rotational levels. For the 3 ν 2 state splittings were observed for K = 3, 6, and 9 because PH 3 is a very nearly spherical top in this state. The magnitude and the J dependence of the observed K = 3 n splittings have been analyzed by using a normal symmetric rotor Hamiltonian and a centrifugal distortion term of the form τ xxxz[(J + 3 + J − 3)J z + J z(J + 3 + J − 3)] 4 .

A study of the electric quadrupole fundamental band of D2 using an infrared difference frequency laser system

Frequency tunable infrared radiation from a difference frequency laser system has been used with a long path cell to study the electric quadrupole vibration–rotation spectrum of the deuterium molecule in the 2.8–3.9 μm region. A ratio method as well as frequency modulation were used to detect the weak absorption. Sub-Doppler linewidths due to collisional narrowing were observed with a D2 pressure of 1 atm. The wavelengths of 11 transitions were measured relative to infrared reference lines, and improved molecular constants for the ν = 0 and 1 states of D2 were determined.

Doppler-limited spectra of the CH stretching fundamentals of formaldehyde

The Doppler-limited spectrum of H 2CO in the 2700 to 3000 cm −1 region has been recorded using a tunable difference-frequency laser system. This region encompasses the ν 1 and ν 5 fundamental CH stretching bands of formaldehyde as well as a number of strongly interacting combination bands. The tunable laser spectrometer affords complete spectral coverage with a calibration precision better than 10 −3 cm −1 for the transition frequencies and with absolute absorption intensity measurements better than 2%. The band centers for the ν 1 and ν 5 vibrations are measured to be 2782.4572 ± 0.0010 cm −1 and 2843.3254 ± 0.0015 cm −1 respectively, independent of the upper-state rotational constants.

High-resolution methane ν_3-band spectra using a stabilized tunable difference-frequency laser system*

Techniques are described for drift compensation and scan calibration of a visible-to-infrared difference-frequency converter enabling high-resolution molecular spectra to be recorded with a precision and reproducibility of 5 × 10−4 cm−1. The capabilities of the spectrometer are illustrated by scans of the tetrahedral fine structure in the ν3 band of methane under low pressure, Doppler-limited and atmospheric pressure, collision-broadened conditions.