There are two big questions we are trying to answer with our research.
The first is rather fundamental, and has been intriguing scientists for
almost 30 years .
The second is of great technological interest for the next generation of space missions:
Helium has been for for a long time (and as far as condensable gases
are concerned, still is) the only system known to exhibit the property of
superfluidity. Scientists like to see if their conclusions are of general
validity, so some of those trying to understand superfluid helium started
looking for another system that could possibly be superfluid. It is only
recently that dilute Bose-Einstein condensated of alkali atoms could be
studied in a laboratory, and we will not deal with them here (the interested
reader can find more in a recent review , or at
so the search was indeed restricted to condensable gases.
Superfluidity is inherently a manifestation of quantum behavior (occurring for a collection of indistinguishable bosons), a convenient measure of which is the De Boer quantum parameter l . After He, H2 has the largest value of l, and is therefore the most natural candidate to look for superfluidity; already 30 years ago it was predicted that H2 should be superfluid below 6 K . Unfortunately, H2 solidifies at 13.8 K, so one needs a high degree of supercooling; despite repeated attempts with rather clever setups, superfluidity in H2 still has to be observed. Clusters are the best candidate to observe superfluidity: they have an estimated temperature of 4.7 K, which is maintained by evaporative cooling . Because of their small size, rapid cooling and absence of nucleation sites, they are most likely liquid.
Part of our research program is linked to the efforts of the Air Force
Office for Scientific Research (AFOSR)
aimed at the discovery of high-performance rocket propellants. The performance
of a propellant is measured by its specific impulse (Isp, defined
which has the dimensions of a time.
Currently, the most efficient propellant is a mixture of liquid oxygen and liquid hydrogen, and has an Isp of about 450 s. The goal of the High Energy Density Matter (HEDM) program is to reach an Isp of 500 s or greater. If this increase seems small, consider that the fuel mass is a substantial fraction of the mass of a rocket (or a missile, or a space shuttle); conversely, the payload is 2-5% of the total mass. Obviously a modest increase in Isp can result in a 100% increase in payload.
The AFOSR effort on higher-performing fuels is currently focused on:
Seeded solid hydrogen is denser than the liquid (requiring less storage space) and, if seeded with the appropriate material (reactive atoms or unstable molecules) has a higher Isp. Incidentally, if solid hydrogen is to be used as a fuel, a fast melting rate is essential; indeed one of the sought properties of the dopants is that it must accelerates melting, e.g. by release of dopant-dopant recombination energy.
Currently, the key issues are:
Admittedly, the typical space shuttle "gas station" will look a little bit different than our experimental apparatus, but as far as items (2) and (3), doped hydrogen clusters are possibly the best system for experimental studies. Each cluster is an isolated system that can be loaded with a controlled number of different atoms/molecules, and interrogated with a variety of spectroscopic techniques.
We produce H2 clusters by expansion in vacuum of high pressure (30 atm) hydrogen gas through a small orifice, or nozzle, (10 µm diameter) kept at a temperature of 60-70 K. Under these conditions, a supersonic beam of H2 is produced; rapid cooling of the gas occurs in the first hundred microns downstream the nozzle, and H2 molecules aggregate into clusters of a few thousands molecules each.
The supersonic beam has a speed of 1200 m/s, so the clusters survive for less than a millisecond (the time it takes to travel from the source to the detector). During this time we (1) load them, (2) excite the doped clusters with our favorite tunable radiation source, (3) detect their interaction with radiation.
So far we have collected the visible fluorescence and beam depletion
spectra of alkali-doped hydrogen clusters. We have found that hydrogen molecules
are very effective at quenching the fluorescence of alkali atoms, to the
extent that a beam depletion approach is necessary to measure the true absorption
spectrum of these complexes .
In a different series of experiments, we have measured the rotationally resolved spectra of the first overtone of several molecules containing the ºCH stretch chromophore, solvated in He clusters, with signal-to-noise ratios of up to 103/Hz1/2. It is currently believed that preservation of rotational coherence is a key signature of superfluidity . If we will be able to observe narrow-line rotationally-resolved spectra of molecules captured by H2 clusters, we will have a strong evidence that they are indeed superfluid.
We are adapting our infrared He cluster machine so that hydrogen clusters
can be produced and investigated as well. Our estimates tell us that we
should be able to detect the absorption spectrum of HCN in the first overtone
of the CH stretch. If the cluster is superfluid, we expect to be able to
resolve the rotational structure. Should the cluster be solid instead, we
might see a series of very sharp lines (similar to those observed for CH4
in solid H2 matrices ), whose splitting pattern would tell
us about the structure and energetics of the trapping site. Since our spectrometer
has an instrumental resolution of about 15 MHz, this circumstance would
be very advantageous in high-resolution studies of exotic species trapped
in a solid matrix.
In all fairness, it is very likely that the HCN or any othe similar molecule will act as a condensation seed and cause the cluster to solidify. A pure cluster is a better candidate to search for superfluidity, if only a suitable spectroscopic transition of the H2 molecule can be found. Moreover, the density of H2 molecules in the beam is obviously much higher than that attainable for dopant molecules. Our plan is to use a Ti:Al2O3 laser and a YAG laser to perform stimulated Raman spectroscopy of pure H2 clusters.
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