John E. Wessel

Design and Evaluation of the First Special Sensor Microwave Imager/Sounder

The first Special Sensor Microwave Imager/Sounder (SSMIS) was launched in October 2003 aboard the Air Force Defense Meteorological Satellite Program (DMSP) F-16 Spacecraft. As originally conceived, the SSMIS integrates the imaging capabilities of the heritage DMSP conically scanning Special Sensor Microwave/Imager sensor with the cross-track microwave sounders Special Sensor Microwave Temperature and Special Sensor Microwave Humidity Sounder, SSM/T-2 into a single conically scanning 24-channel instrument with extended sounding capability to profile the mesosphere. As such, the SSMIS represents the most complex operational satellite passive microwave imager/sounding sensor flown while, at the same time, offering new and challenging capabilities associated with radiometer channels having common fields of view, uniform polarizations, and fixed spatial resolutions across the active scene scan sector. A comprehensive end-to-end calibration/validation (cal/val) of the first SSMIS initiated shortly after launch was conducted under joint sponsorship by the DMSP and the Navy Space and Warfare Systems Command. Herein, we provide an overview of the SSMIS instrument design, performance characteristics, and major cal/val results. Overall, the first SSMIS instrument exhibits remarkably stable radiometer sensitivities, meeting requirements with considerable margin while providing high-quality imagery for all channels. Two unanticipated radiometer calibration anomalies uncovered during the cal/val-sun intrusion into the warm-load calibration target and antenna reflector emissions-required significant attention during the cal/val program. In particular, the tasks of diagnosing the root cause(s) of these anomalies as well as the development of ground processing software algorithms to mitigate their impact on F-16 SSMIS and hardware fixes on future instruments necessitated the construction of extensive analysis and simulation tools. The lessons learned from the SSMIS cal/val and the associated analysis tools are expected to play an important role in the design and performance evaluation of future passive microwave imaging and sounding instruments as well as guiding the planning and development of future cal/val programs.

Saturated optical nonresonant emission spectroscopy (SONRES) for atomic detection

We have demonstrated the extremely small detection limits that can be achieved in a high‐pressure environment by the combination of saturated absorption and nonresonance emission spectroscopy. The method of SONRES exploits efficient energy transfer between excited atomic states at high pressures and intense optical fields available from tunable lasers. An estimated 104 atoms/cm3 of sodium were monitored in a flame; the corresponding atomic concentration was 1 part in 1015. A detection limit of ∼10 atoms/cm3 was recorded for sodium in argon. The ultimate detection limit for this method is ∼10−4 atoms.

Picosecond transient reflectivity of unpinned gallium arsenide (100) surfaces

Surface recombination was measured for photowashed and unwashed GaAs using picosecond transient photoreflectance methods. The results for the washed surfaces clearly demonstrate slow surface recombination that is accurately described by an ambipolar diffusion model. The fast decay observed for unwashed samples implies rapid surface recombination involving a more complex mechanism.

Resonant two‐photon ionization spectroscopy of naphthalene clusters

Mass resolved 2−photon ionization spectra of naphthalene clusters are reported. Differences are observed between spectral structures associated with totally symmetric and nontotally symmetric monomer vibrational modes in the excited state. This provides unique information concerning resonance interactions and geometry of clusters.(AIP)

Resonance ion dissociation spectroscopy of naphthalene ions prepared in a supersonic expansion

Resonance ion dissociation (RID) spectra are reported for cations of naphthalene and 2‐methylnaphthalene cooled in a supersonic beam. Discrete vibronic resonances were observed in the ultraviolet region of both ions. A discrete red system was also observed for 2‐methylnaphthalene that is subject to strong degenerate vibronic interaction with an underlying quasicontinuum associated with a lower energy electronic transition. This leads to effective power broadening of the RID spectra at moderately low laser intensities. The red two‐photon dissociation process of 2‐methylnaphthalene was successfully modeled by classical rate equations applied to a four level system, consisting of a ground state, the directly excited state, a lower energy excited state, and a final dissociative state.

Spectroscopic evidence for high symmetry in (benzene)13

The (benzene)13 cluster and its isotopic derivatives have been formed in a dilute benzene/helium jet, and investigated by mass‐selective R2PI spectroscopy in the region of the B2u ← A1g 000 and 610 transitions. Each band exhibits resolved fine structure, which differs greatly between forbidden (000) and allowed (610) bands. The main features, identified by spectral shift, are proposed to correspond to distinct molecular sites, whose symmetries are deduced by comparing spectra of the allowed and induced vibronic bands. The isotopic substitution of one C6H6 molecule into an otherwise deuterated cluster results in a considerable spectral simplification, due in part to highly nonrandom labelling. The results appear to rule out the crystallographic structure, while being consistent with the compact noncrystallographic structure computed by van de Waal.

Infrared detection by an atomic vapor quantum counter

An efficient technique for the detection of nartowband infrared radiation by up-conversion in an optically saturated atomic vapor is described. The strong transition cross sections provided by the resonance interactions combined with the narrow Doppler spectral response profile contribute to the potential to approach quantum-noise-limited performance. Experimental results confirm the model of the detection process for sodium transitions at 1.48, 2.34, and 3.42 μ. The predicted ultimate noise equivalent power (NEP) of this device is less than 10-17W/Hz1/2in the 1.5-5-\mu spectral region.

Single-atom detection by SONRES

The new method of atomic fluorescence detection, called saturated optical nonresonant emission spectroscopy (SONRES), has been modeled for a three-level atom, and experiments on sodium have been conducted that support the model. A rate equation analysis yielded expressions for excited-state atomic populations and saturation intensity. The detection of sodium in buffer gases that promote collisional transfer of excitation between 32P 1/2 → 32p 3/2 both without quenching the fluorescence emission and with quenching was considered. Experimental results are presented for sodium in argon. At -25°C, approximately 180 atoms/cm3were monitored with a S/N of ∼ 15 representing detection at the level of one part in 1017. The signal at this temperature was generated by less than a single atom in the laser beam.

Identification of a high energy state of I2 by ion dip spectroscopy

A new high energy electronic state of I2 has been characterized by means of ion dip spectroscopy, a two‐color multiphoton ionization technique. The method yields rotationally resolved spectra analogous to excited state absorption spectra. The new state is characterized by an extensive 37 cm−1 vibrational progression, an internuclear separation of about 5.3±0.2 A, and a term energy about 70 000 cm−1, near the ionization limit. Bonding is significantly weaker than in the previously investigated ion‐pair states. The new results help to establish the pathway of the multiphoton ionization process in I2.

Ultrasensitive detection of aromatic hydrocarbons by two‐photon photoionization

An optimized multiphoton photoionization detection system has been applied to monitor aromatic‐hydrocarbon vapor density as a function of temperature. The density curves established for naphthalene by this procedure permit estimation of a detection limit of 5×104 molecules/cm3 in a nitrogen buffer gas. With slight modification this method woud be capable of single‐molecule detection limits.