Squeezing a helium nanodroplet with a Rydberg electron.

We have investigated, by means of density functional theory, the structure of a "scolium", that is, an electron circulating around a positively charged 4He nanodroplet, temporarily prevented from neutralization by the helium-electron repulsion. The positive ion core resides in the center of the nanodroplet where, as a consequence of electrostriction, a strong increase in the helium density with respect to its bulk value occurs. The electron enveloping the 4He cluster exerts an additional electrostatic pressure which further increases the local 4He density around the ion core. We argue that under such pressure, sufficiently small 4He nanodroplets may turn solid. The stability of a scolium with respect to electron-ion recombination is investigated.

Martian CH(4): sources, flux, and detection.

Recent observations have detected trace amounts of CH(4) heterogeneously distributed in the martian atmosphere, which indicated a subsurface CH(4) flux of ~2 x 10(5) to 2 x 10(9) cm(2) s(1). Four different origins for this CH(4) were considered: (1) volcanogenic; (2) sublimation of hydrate- rich ice; (3) diffusive transport through hydrate-saturated cryosphere; and (4) microbial CH(4) generation above the cryosphere. A diffusive flux model of the martian crust for He, H(2), and CH(4) was developed based upon measurements of deep fracture water samples from South Africa. This model distinguishes between abiogenic and microbial CH(4) sources based upon their isotopic composition, and couples microbial CH(4) production to H(2) generation by H(2)O radiolysis. For a He flux of approximately 10(5) cm(2) s(1) this model yields an abiogenic CH(4) flux and a microbial CH(4) flux of approximately 10(6) and approximately 10(9) cm(2) s(1), respectively. This flux will only reach the martian surface if CH(4) hydrate is saturated in the cryosphere; otherwise it will be captured within the cryosphere. The sublimation of a hydrate-rich cryosphere could generate the observed CH(4) flux, whereas microbial CH(4) production in a hypersaline environment above the hydrate stability zone only seems capable of supplying approximately 10(5) cm(2) s(1) of CH(4). The model predicts that He/H(2)/CH(4)/C(2)H(6) abundances and the C and H isotopic values of CH(4) and the C isotopic composition of C(2)H(6) could reveal the different sources. Cavity ring-down spectrometers represent the instrument type that would be most capable of performing the C and H measurements of CH(4) on near future rover missions and pinpointing the cause and source of the CH(4) emissions.

Dynamics of the 1 3 pi g state of K2 on helium nanodroplets.

Fluorescence following optical excitation of the 1 3 sigma u+ state of K2 prepared on helium nanodroplets to the predissociative 1 3 pi g state yields molecular emission from both the (B)1 1 pi u and (A)1 1 sigma u+ K2 states as well as atomic emission from the expected 4 2P3/2, 1/2-->4 2S1/2 dissociation channel. A approximately 12 cm-1 red shift is observed in the molecular emission excitation spectrum compared to the atomic emission excitation spectrum. Time-correlated photon counting measurements demonstrate the rise time for both atomic and molecular products to be < 80 ps, independent of vibrational level excited. This lifetime is interpreted as the total depopulation time for the optically excited 1 3 pi g state, which is dominated by intersystem crossing at low vibrational energy and by predissociation at the highest vibrational level. It is deduced that the timescale for intersystem crossing must be of the order of 10 ps. Symmetry restrictions for the isolated K2 imply that the intersystem crossing from the 1 3 pi g state to the (B)1 1 pi u and (A)1 1 sigma u+ states must be induced by interaction with the helium nanodroplet.

Doppler-free nonlinear absorption in ethylene by use of continuous-wave cavity ringdown spectroscopy.

We report what we believe to be the first systematic study of Doppler-free, nonlinear absorption by use of cavity ringdown spectroscopy. We have developed a variant of cavity ringdown spectroscopy for the mid-infrared region between 9 and 11 microm, exploiting the intracavity power buildup that is possible with continuous-wave lasers. The infrared source consists of a continuous-wave CO2 laser with 1-mW tunable infrared sidebands that couple into a high-finesse stable resonator. We tune the sideband frequencies to observe a saturated, Doppler-free Lamb dip in the nu7, 11(1,10) <-- 11(2,10) rovibrational transition of ethylene (C2H4). Power studies of the Lamb dip are presented to examine the intracavity effects of saturation on the Lamb-dip linewidth, the peak depth, and the broadband absorption.

Predicting long-term disability in low back injured workers presenting to a spine consultant.

Low back pain (LBP) is the most common, costly, and disabling musculoskeletal condition. Although most LBP patients recover within two months, 2-3% eventually develop disabling chronic low back pain (DCLBP). Due to the prevalence of DCLBP problems, models have been developed to predict which acute low back pain patients are predisposed to the problems associated with this condition. Many see the development of these models as a first step that must be taken before useful approaches for containing and reducing the problem can be conceptualized, implemented, and tested. A recent publication by Cats-Baril and Frymoyer considered this specific problem. While the results of their study indicate considerable success in predicting DCLBP patients, the high prediction rates they obtained may be spurious because of the characteristics of their sampled patient population in conjunction with some of the predictors they found useful in identifying DCLBP patients. The purpose of the present study was to focus on the crucial patient population (i.e., acute LBP patients who perceive their problem as work-related and who have been unable to work for more than two but less than six weeks), and evaluate the ability of various personal, medical, occupational, and psychological factors to predict predisposition to DCLBP. Fifty-five patients referred by occupational physicians were evaluated and followed successfully for at least 6 months. Patients in the study were given a physical examination that included Spratt et al's assessment of pain behavior. They were then asked to fill out an extensive battery of self-report questionnaires, addressing issues associated with personal demographics, health history, work requirements, job satisfaction, injury information, and pain/function factors. At the 6-month follow-up, a structured telephone interview was used to obtain outcome information regarding patient status, including ability to return to work and general outcomes of treatment. Average patient age was 37.2 years (range, 22-57) and 67% of the patients were male. On average, patients had been unable to work for approximately 4 weeks when initially surveyed. Overall, 12.7% of the patients returned to work within 1 month of injury, 40% returned within 2 months, 54.5% within 3 months, 69% within 4 months, 74.5% within 5 months, 76.3% within 6 months, 80% within 7 months, and 83.6% after 7 months. Approximately 16% never successfully returned to work within the follow-up period of this study. DCLBP was found to be correlated only with marital status, as married patients returned to work more quickly than single patients (P < 0.01).(ABSTRACT TRUNCATED AT 400 WORDS)