Frank C. DeLucia

Gauche Ethyl Alcohol: Laboratory Assignments and Interstellar Identification

Ethyl alcohol (ethanol) is known to possess a pair of closely spaced excited torsional substates (gauche+, gauche-) at an energy of approximately 57 K above the ground (trans) torsional substate. We report an extended analysis of some gauche- -gauche+ Q-branch (ΔJ = 0) transitions with a three-substate fixed frame axis method (FFAM) Hamiltonian. Our approach accounts for complex trans-gauche interactions for the first time. In addition, we are able to obtain intensities for perturbed rotational transitions, and to determine the trans to gauche+ separation to be 1185399.1 MHz. A complete ground state rotational-torsional partition function accounting for the previously neglected gauche substates is presented. Based on our analysis, a total of 14 U lines obtained towards Orion KL can now be assigned to gauche substates of ethanol. Analysis of these lines yields a rotational temperature of 223 K and a total (trans + gauche) column density of 7.0 × 1015 cm-2. The column density is in reasonable agreement with the recent value of 2-3 × 1015 cm-2 based on observations of trans-ethanol by Ohishi et al., although there is some disparity in the rotational temperatures. Eight additional U lines in the literature are assigned to transitions of gauche ethanol.

Concealed weapon identification using terahertz imaging sensors

Terahertz imaging sensors are being considered for providing a concealed weapon identification capability for military and security applications. In this paper the difficulty of this task is assessed in a systematic way. Using imaging systems operating at 640 GHz, high resolution imagery of possible concealed weapons has been collected. Information in this imagery is removed in a controlled and systematic way and then used in a human observer perception experiment. From the perception data, a calibration factor describing the overall difficulty of this task was derived. This calibration factor is used with a general model of human observer performance developed at the US Army Night Vision and Electronic Sensors Directorate to predict the task performance of observers using terahertz imaging sensors. Example performance calculations for a representative imaging sensor are shown.

Variable temperature pressure broadening of the 41.4-32.1 transition of H2O by O2 and N2

Abstract The O2 and N2 pressure-broadening parameters of the 41,4-32,1 rotational transition in the ground vibrational state of H2O have been measured in the temperature range between 100 and 520 K. Above 250 K the measurements were made in an equilibrium cell. Below 250 K a cell, which uses collisional cooling to circumvent the temperature limits imposed by the vapor pressure of the sample gas, was used. The data were fitted to an exponential temperature-dependence for data above 150 K with resultant n values of 0.81(3) for O2 and 0.70(3) for N2, where the entries in parentheses represent the error in the last significant figure of the parameter. Below 150 K the measured pressure-broadening parameters are smaller than those calculated using these values of n.

Observation and analysis of high-J o1−e1 inter-state transitions in CH2DOH

We report the observation and assignments of several series of transitions in CH2DOH, including a new and intense series of Q-branch c-type transitions between the e1 and o1 torsional substates through high values of the rotational quantum number J. Other transitions assigned are c-type R-branch and P-branch transitions between these two substates and a-type lines within the e1 and o1 substates. The assignments were facilitated by initial analysis of the strong Q-branch series, as recorded by the fast scan submillimeter spectroscopy technique (FASSST). The assigned lines of CH2DOH include the first inter-state transitions in the sub-millimeter-wave region and the first to possess high J-values. The completeness of the data generated by FASSST and the success of a simple power series analysis suggest that many parts of the spectrum of CH2DOH may be far more tractable than previously believed. The data should be useful in the development of a full Hamiltonian and in the assignment of astronomical emission f...

Hydrogen and Helium Pressure Broadening of H2S Between 2 and 600 K

Abstract The hydrogen (H 2 ) and helium (He) pressure-broadening cross sections of the 1 10 ← 1 01 transition of H 2 S have been measured between 2 and 600 K. This more than two decade range in temperature makes possible the consideration of these collisions as a spectroscopic process in which the translational energy plays the role of the spectroscopic variable. Although in the semiclassical region the pressure broadening due to H 2 and He is similar, at low temperature large qualitative differences occur, resulting in almost an order of magnitude larger cross section for collisions with H 2 below 10 K.

Modulating and scanning the mode-lock frequency of an 800-MHz femtosecond Ti:sapphire laser

An 800-MHz self-mode-locked femtosecond Ti:sapphire laser has been developed whose mode-lock frequency can be robustly scanned and modulated. The laser is based on the three-element design of Ramaswamy-Paye and Fujimoto [Opt. Lett. 19, 1756 (1994)]. By translation and modulation of the position of the prismatic output coupler, the mode-lock frequency can be reliably scanned more than 1% and modulated at 80 Hz with a deviation of 2.5 kHz without interrupting the mode lock, changing the pulse length, or inducing significant amplitude modulation. An application in tunable high-resolution terahertz spectroscopy is also demonstrated.

Double resonance measurements of far infrared lasing transitions in methanol

Abstract In this work, precise frequencies have been measured for two important far-infrared (FIR) laser transitions in methanol. These are optically pumped by the 9P(16) CO 2 laser line at an offset of +64 MHz from the line center, using infrared (IR)-millimetrewave (MMW) double resonance (DR) techniques. The two measured FIR lines are at 525,427.902 and 477,790.879 MHz, with an estimated precision of ±50 kHz. Another line has been observed at 573,025.151 MHz. It corresponds to an absorption transition whose lower level is the upper pumped level. The measurements provide a valuable confirmation of the assignments of the IR as well as the FIR transitions. In addition, accurate frequencies for several transitions have been determined in the ground vibrational state in the range 109–127 GHz and provide assignments for additional microwave lines.

Fast-scanning spectroscopic method for the submillimeter: the FASSST spectrometer

A new FAst Scan Submillimeter Spectroscopic Technique (FASSST) is described. It uses voltage tunable Backward Wave Oscillators (BWOs) as primary sources of radiation. In contrast to the more traditional phase or frequency lock techniques, it uses fast scan (approximately 105 Doppler limited resolution elements/sec) and optical calibration methods. Its attributes include (1) absolute frequency calibration to approximately 1/10 of a Doppler limited linewidth (less than 0.1 MHz), (2) high sensitivity, (3) the ability to measure many thousands of lines/sec, and (4) simplicity. This system is made possible by (1) the excellent short term spectral purity of the broadly (approximately 100 GHz) tunable BWOs, (2) a very low noise, rapidly scannable high voltage power supply, (3) fast data acquisition, and (4) software capable of automated calibration and spectral line measurement.

trans-Ethyl Methyl Ether: Laboratory Measurements and Analysis

High-resolution measurements of rotational-torsional transitions of ethyl methyl ether (EME) were performed in its vibrational-torsional ground state at frequencies up to 350 GHz [1]. The new results correspond to about 1000 new observed transitions including internal rotation splitting of each transition due to torsion of the two methyl groups. A simultaneous fit to experimental accuracy of these and approximately 100 previously assigned transitions (see [2]) allows reliable prediction of additional transition frequencies up to 400 GHz, based on a newly calculated parameter set. The new data set should enable the astrophysical detection of ethyl methyl ether in the interstellar medium. Ethyl methyl ether was tentatively detected in hot cores which are quiescent high-density sources associated with regions of high-mass star formation by Charnley et al. [3].