High resolution spectroscopy in the microwave and far infrared

High resolution rotational spectroscopy has long been central to remote sensing techniques in atmospheric sciences and astronomy. As such, laboratory measurements must supply the required data to make direct interpretation of data for instruments which sense atmospheres using rotational spectra. Spectral measurements in the microwave and far infrared regions are also very powerful tools when combined with infrared measurements for characterizing the rotational structure of vibrational spectra. In the past decade new techniques were developed which have pushed high resolution spectroscopy into the wavelength region between 25 micrometers and 2 mm. Techniques to be described include: (1) harmonic generation of microwave sources, (2) infrared laser difference frequency generation, (3) laser sideband generation, and (4) ultrahigh resolution interferometers

Method and means for generation of tunable laser sidebands in the far-infrared region

A method for generating tunable far-infrared radiation is described. The apparatus includes a Schottky-barrier diode which has one side coupled through a conductor to a waveguide that carries a tunable microwave frequency; the diode has an opposite side which is coupled through a radiating whisker to a bias source. Infrared light is directed at the diode, and infrared light with tunable sidebands is radiated by the whisker through an open space to a reflector. The original infrared is separated from a tunable infrared sideband by a polarizing Michelson interferometer

Submillimeter-Wave Measurements and Analysis of the Ground and ν2 = 1 States of Water

In order to facilitate further studies of water in the interstellar medium, the envelopes of late-type stars, jets, and shocked regions, the frequencies of 17 newly measured H_2 ^(16)O transitions between 0.841 and 1.575 THz are reported. A complete update of the available water line frequencies and a detailed calculation of unmeasured rotational transitions and transition intensities as a function of temperature are presented for the ground and ν_2 = 1 state levels below 3000 cm^(-1) of excitation energy. The new terahertz transitions were measured with a recently developed laser difference frequency spectrometer. Six of these transitions arise from the ν_2 = 1 state, and the other 11 are in the ground state; all have lower state energies from 700 to 1750 cm^(-1) and should be accessible to Stratospheric Observatory For Infrared Astronomy (SOFIA) through the atmosphere. The transitions near 0.850 THz are accessible from the ground with existing receivers. Observations of the newly measured ν_2 = 1 state transitions, which include the 1_(1, 1)-0_(0, 0) fundamental at 1.2057 THz and five other very low J transitions, should provide valuable insights into role played by the ν2 = 1 state in the cooling dynamics of jets, shocks, masers, and strongly infrared-pumped regions. The line list is presented to assist in the planning of observational campaigns with the Far-Infrared Space Telescope (FIRST) and other proposed space missions with which a full suite of water observations can be carried out

A High Power Frequency-Stabilized Tunable Two-Frequency Diode Laser System for the Generation of Coherent THz-Waves by Photomixing

A tunable two-frequency high-power diode laser system at 850 nm for terahertz (Thz)-wave generation by photomixing in low-temperature-grown GaAs photo-conductors has been developed. The difference frequency is obtained through a three laser system, where two lasers are locked to different orders of a Fabry-Perot cavity and a third is offset-locked to the first, The difference-frequency signal is generated by the offset laser and the other cavity-locked laser. The spectral purity of the beat note is better than 1 MHz. The maximum output power of -500 mW was obtained by using the master oscillator power amplifier (MOPA) technique, simultaneous injection of two seed frequencies with a single semiconductor optical amplifier. Here we report the generation of THz waves and spectroscopy of acetonitrile as proof of concept

A tunable cavity-locked diode laser source for terahertz photomixing

An all solid-state approach to the precise frequency synthesis and control of widely tunable terahertz radiation by differencing continuous-wave diode lasers at 850 nm is reported in this paper. The difference frequency is synthesized by three fiber-coupled external-cavity laser diodes. Two of the lasers are Pound-Drever-Hall locked to different orders of a Fabry-Perot (FP) cavity, and the third is offset-frequency locked to the second of the cavity-locked lasers using a tunable microwave oscillator. The first cavity-locked laser and the offset-locked laser produce the difference frequency, whose value is accurately determined by the sum of an integer multiple of the free spectral range of the FP cavity and the offset frequency. The dual-frequency 850-nm output of the three laser system is amplified to 500 mW through two-frequency injection seeding of a single semiconductor tapered optical amplifier. As proof of precision frequency synthesis and control of tunability, the difference frequency is converted into a terahertz wave by optical-heterodyne photomixing in low-temperature-grown GaAs and used for the spectroscopy of simple molecules. The 3-dB spectral power bandwidth of the terahertz radiation is routinely observed to be ≾1 MHz. A simple, but highly accurate, method of obtaining an absolute frequency calibration is proposed and an absolute calibration of 10^(-7) demonstrated using the known frequencies of carbon monoxide lines between 0.23-1.27 THz

Two-frequency MOPA diode laser system for difference-frequency generation of coherent THz waves

We developed a tunable, cavity-locked diode laser source at 850 nm for difference-frequency generation of coherent THz- waves. The difference frequency is synthesized by three fiber-coupled external-cavity diode lasers, where tow of the lasers are locked to adjacent modes of an ultra-stable Fabry-Perot cavity and the third laser is offset-phase- locked to the second cavity-locked laser using a tunable microwave oscillator. The first cavity-locked laser and the offset-locked laser produces the difference frequency, whose value is precisely determined by sum of integer multiple of free spectral range of the Fabry-Perot cavity and the offset frequency. The difference-frequency signal is amplified to 500 mW by the master oscillator power amplifier technique, simultaneous two-frequency injection-seeding with a single semiconductor optical amplifier. Here we demonstrate the difference-frequency generation of THz waves with the low- temperature-grown GaAs photomixers and its application to high-resolution spectroscopy of simple molecules. An absolute frequency calibration was carried out with an accuracy of approximately 10^(-7) using CO lines in the THz region

Direct measurement of the fundamental rotational transitions of the OH radical by laser sideband spectroscopy

We report for the first time the direct (zero-field) spectra of the fundamental rotational transitions of the OH radical in its Ω = 3/2 and 1/2 states at 2509.9 and 1834.7 GHz using a recently developed far-infrared laser sideband spectrometer. These measurements have verified and refined the predictions of previous laser magnetic resonance (LMR) work, thereby confirming the far-infrared detection of interstellar OH. The increased accuracy of these direct measurements will be useful to future astronomical and atmospheric studies of these important transitions

The Covalent Interaction between Dihydrogen and Gold: a Rotational Spectroscopic Study of H₂-AuCl

The pure rotational transitions of H2-AuCl have been measured using a pulsed-jet cavity Fourier transform microwave spectrometer equipped with a laser ablation source. The structure was found to be T-shaped, with the H-H bond interacting with the gold atom. Both 35Cl and 37Cl isotopologues have been measured for both ortho and para states of H2. Rotational constants, quartic centrifugal distortion constants, and nuclear quadrupole coupling constants for gold and chlorine have been determined. The use of the nuclear spin-nuclear spin interaction terms Daa, Dbb, and Dcc for H2 were required to fit the ortho state of hydrogen, as well as a nuclear-spin rotation constant Caa. The values of the nuclear quadrupole coupling constant of gold are Xaa=-817.9929(35) MHz, Xbb=504.0(27) MHz, and Xcc=314.0(27). This is large compared to the eQq of AuCl, 9.63 312(13) MHz, which indicates a strong, covalent interaction between gold and dihydrogen

Polar Northern Hemisphere Middle Atmospheric Influence due to Energetic Particle Precipitation in January 2005

Solar eruptions and geomagnetic activity led to energetic particle precipitation in early 2005, primarily during the January 16-21 period. Production of OH and destruction of ozone have been documented due to the enhanced energetic solar proton flux in January 2005 [e.g., Verronen et al., Geophys. Res. Lett.,33,L24811,doi:10.1029/2006GL028115, 2006; Seppala et al., Geophys. Res. Lett.,33,L07804, doi:10.1029/2005GL025571,2006]. These solar protons as well as precipitating electrons also led to the production of NO(x) (NO, NO2). Our simulations with the Whole Atmosphere Community Climate Model (WACCM) show that NO(x) is enhanced by 20-50 ppbv in the polar Northern Hemisphere middle mesosphere (approx.60-70 km) by January 18. Both the SCISAT-1 Atmospheric Chemistry Experiment (ACE) NO(x) measurements and Envisat Michelson Interferometer for Passive Atmospheric Sounding (MIP AS) nighttime NO2 observations show large increases during this period, in reasonable agreement with WACCM predictions. Such enhancements are considerable for the mesosphere and led to simulated increases in polar Northern Hemisphere upper stratospheric odd nitrogen (NO(y)) of2-5 ppbv into February 2005. The largest ground level enhancement (GLE) of solar cycle 23 occurred on January 20, 2005 with a neutron monitor increase of about 270 percent [Gopalswamy et al., 29th International Cosmic Ray Conference, Pune,00,101-104,2005]. We found that protons of energies 300 to 20,000 MeV, not normally included in our computations, led to enhanced stratospheric NO(y) of less than 1 percent as a result of this GLE. The atmospheric impact of precipitating middle energy electrons (30-2,500 keV) during the January 16-21, 2005 period is also of interest, and an effort is ongoing to include these in WACCM computations. This presentation will show both short- and longer-term changes due to the January 2005 energetic particle precipitation

Two-frequency MOPA diode laser system for difference-frequency generation of coherent THz waves

We developed a tunable, cavity-locked diode laser source at 850 nm for difference-frequency generation of coherent THz- waves. The difference frequency is synthesized by three fiber-coupled external-cavity diode lasers, where tow of the lasers are locked to adjacent modes of an ultra-stable Fabry-Perot cavity and the third laser is offset-phase- locked to the second cavity-locked laser using a tunable microwave oscillator. The first cavity-locked laser and the offset-locked laser produces the difference frequency, whose value is precisely determined by sum of integer multiple of free spectral range of the Fabry-Perot cavity and the offset frequency. The difference-frequency signal is amplified to 500 mW by the master oscillator power amplifier technique, simultaneous two-frequency injection-seeding with a single semiconductor optical amplifier. Here we demonstrate the difference-frequency generation of THz waves with the low- temperature-grown GaAs photomixers and its application to high-resolution spectroscopy of simple molecules. An absolute frequency calibration was carried out with an accuracy of approximately 10^(-7) using CO lines in the THz region