2,676 research outputs found
The abundances of ethane to acetylene in the atmospheres of Jupiter and Saturn
The present determination of the stratospheric abundances of ethane and acetylene on Jupiter and Saturn on the basis of IR spectra near 780/cm uses atmospheric models whose thermal and density profiles have constant mixing ratios. The ratio of ethane to acetylene is noted to be insensitive to model atmosphere assumptions; it is 55 + or - 31 for Jupiter and 23 + or - 12 where model mixing ratios are uniform. Atmospheric model density profiles adapted from theoretical photochemical models are noted to also yield a higher ethane/acetylene ratios for Jupiter
A search for varying fundamental constants using Hz-level frequency measurements of cold CH molecules
Many modern theories predict that the fundamental constants depend on time,
position, or the local density of matter. We develop a spectroscopic method for
pulsed beams of cold molecules, and use it to measure the frequencies of
microwave transitions in CH with accuracy down to 3 Hz. By comparing these
frequencies with those measured from sources of CH in the Milky Way, we test
the hypothesis that fundamental constants may differ between the high and low
density environments of the Earth and the interstellar medium. For the fine
structure constant we find \Delta\alpha/\alpha = (0.3 +/- 1.1)*10^{-7}, the
strongest limit to date on such a variation of \alpha. For the
electron-to-proton mass ratio we find \Delta\mu/\mu = (-0.7 +/- 2.2) * 10^{-7}.
We suggest how dedicated astrophysical measurements can improve these
constraints further and can also constrain temporal variation of the constants.Comment: 8 pages, 3 figure
A strong 3.4 micron emission feature in comet Austin 1989c1
High resolution 2.8-4.0 micron spectra of the 'new' comet Austin 1989c1, taken on 15-16 May 1990 confirm the presence of the broad emission features around 3.4 and 3.52 micron seen in a number of bright comets and ascribed to organic material. Both the 3.4 micron band strength and the 3.52/3.36 micron flux ratios are among the largest so far observed. The data are consistent with the relationship between band strength and water production rate that was recently derived. Excess emission at 3.28 and 3.6 micron cannot be unambiguously identified as features due to the poor signal-to-noise ratio
Nonadiabatic transitions in a Stark decelerator
In a Stark decelerator, polar molecules are slowed down and focussed by an
inhomogeneous electric field which switches between two configurations. For the
decelerator to work, it is essential that the molecules follow the changing
electric field adiabatically. When the decelerator switches from one
configuration to the other, the electric field changes in magnitude and
direction, and this can cause molecules to change state. In places where the
field is weak, the rotation of the electric field vector during the switch may
be too rapid for the molecules to maintain their orientation relative to the
field. Molecules that are at these places when the field switches may be lost
from the decelerator as they are transferred into states that are not focussed.
We calculate the probability of nonadiabatic transitions as a function of
position in the periodic decelerator structure and find that for the
decelerated group of molecules the loss is typically small, while for the
un-decelerated group of molecules the loss can be very high. This loss can be
eliminated using a bias field to ensure that the electric field magnitude is
always large enough. We demonstrate our findings by comparing the results of
experiments and simulations for the Stark deceleration of LiH and CaF
molecules. We present a simple method for calculating the transition
probabilities which can easily be applied to other molecules of interest.Comment: 12 pages, 9 figures, minor revisions following referee suggestion
Stark deceleration of lithium hydride molecules
We describe the production of cold, slow-moving LiH molecules. The molecules
are produced in the ground state using laser ablation and supersonic expansion,
and 68% of the population is transferred to the rotationally excited state
using narrowband radiation at the rotational frequency of 444GHz. The molecules
are then decelerated from 420m/s to 53m/s using a 100 stage Stark decelerator.
We demonstrate and compare two different deceleration modes, one where every
stage is used for deceleration, and another where every third stage decelerates
and the intervening stages are used to focus the molecules more effectively. We
compare our experimental data to the results of simulations and find good
agreement. These simulations include the velocity dependence of the detection
efficiency and the probability of transitions between the weak-field seeking
and strong-field seeking quantum states. Together, the experimental and
simulated data provide information about the spatial extent of the source of
molecules. We consider the prospects for future trapping and sympathetic
cooling experiments.Comment: 14 pages, 6 figures; minor revisions following referee suggestion
Microwave spectroscopy of Lambda-doublet transitions in the ground state of CH
The Lambda-doublet transitions in CH at 3.3 and 0.7 GHz are unusually
sensitive to variations in the fine-structure constant and the
electron-to-proton mass ratio. We describe methods used to measure the
frequencies of these transitions with Hz-level accuracy. We produce a pulsed
supersonic beam of cold CH by photodissociation of CHBr3, and we measure the
microwave transition frequencies as the molecules propagate through a
parallel-plate transmission line resonator. We use the molecules to map out the
amplitude and phase of the standing wave field inside the transmission line. We
investigate velocity-dependent frequency shifts, showing that they can be
strongly suppressed through careful timing of the microwave pulses. We measure
the Zeeman and Stark effects of the microwave transitions, and reduce
systematic shifts due to magnetic and electric fields to below 1 Hz. We also
investigate other sources of systematic uncertainty in the experiment.Comment: 27 pages, 12 figure
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