Robert deCarvalho

Magnetic trapping of calcium monohydride molecules at millikelvin temperatures

Recent advances in the magnetic trapping and evaporative cooling of atoms to nanokelvin temperatures have opened important areas of research, such as Bose–Einstein condensation and ultracold atomic collisions. Similarly, the ability to trap and cool molecules should facilitate the study of ultracold molecular physics and collisions; improvements in molecular spectroscopy could be anticipated. Also, ultracold molecules could aid the search for electric dipole moments of elementary particles. But although laser cooling (in the case of alkali metals,,) and cryogenic surface thermalization (in the case of hydrogen,) are currently used to cool some atoms sufficiently to permit their loading into magnetic traps, such techniques are not applicable to molecules, because of the latters complex internal energy-level structure. (Indeed, most atoms have resisted trapping by these techniques.) We have reported a more general loading technique based on elastic collisions with a cold buffer gas, and have used it to trap atomic chromium and europium,. Here we apply this technique to magnetically trap a molecular species—calcium monohydride (CaH). We use Zeeman spectroscopy to determine the number of trapped molecules and their temperature, and set upper bounds on the cross-sectional areas of collisional relaxation processes. The technique should be applicable to many paramagnetic molecules and atoms.

High-Flux Beam Source for Cold, Slow Atoms or Molecules

We demonstrate and characterize a high-flux beam source for cold, slow atoms or molecules. The desired species is vaporized using laser ablation, then cooled by thermalization in a cryogenic cell of buffer gas. The beam is formed by particles exiting a hole in the buffer gas cell. We characterize the properties of the beam (flux, forward velocity, temperature) for both an atom (Na) and a molecule (PbO) under varying buffer gas density, and discuss conditions for optimizing these beam parameters. Our source compares favorably to existing techniques of beam formation, for a variety of applications.

Zeeman spectroscopy of CaH molecules in a magnetic trap

In a recent experiment [Weinstein et al., Nature 395, 148 (1998)] we magnetically trapped 108 ground-state calcium monohydride molecules, CaH(X 2Σ,v″=0, J″=0). The molecules were prepared by laser ablation of a solid sample of CaH2 and loaded via thermalization with a cold (<1 K) 3He buffer gas. The magnetic trap was formed by superconducting coils arranged in the anti-Helmholtz configuration. The detection was done by laser fluorescence spectroscopy excited at 635 nm (in the B 2Σ,v′=0−X 2Σ,v″=0 band) and detected at 692 nm (within the B,v′=0−X,v″=1 band). Both a photomultiplier tube and a CCD camera were used. Due to the thermalization of molecular rotation, only a transition from the lowest rotational state could be detected at zero field, N′=1, J′=3/2←N″=0, J″=1/2. In the magnetic field this rotational transition splits into two features, one shifted towards lower and one towards higher frequencies. The measured shifts are linear in field strength and indicate a small difference (0.02 μB) in the magnet...

Spectroscopy of buffer-gas cooled vanadium monoxide in a magnetic trapping field

Spectroscopy of buffer-gas cooled vanadium monoxide (VO) is performed in the presence of a magnetic trapping field and at low field. VO is created via laser ablation. A helium buffer gas, chilled by a dilution refrigerator, cools 1012 VO molecules to 1.8±0.2 K within 10 ms. The measured rotational temperature is 1.5±0.8 K. Spatially resolved Zeeman spectra allow the magnetic broadening terms of several optical transitions to be determined. The density of VO decays with a characteristic time of 60 ms, thus precluding the observation of trapping.

Towards magnetic trapping of molecules

The advent of buffer-gas loaded magnetic traps for atoms has opened the possibility of trapping paramagnetic molecules. We survey our results on the loading, trapping and spectroscopy of Eu atoms that demonstrated the technique. The principles governing molecular trapping considering in particular the O2 and NO systems are outlined. The trapping of molecules should prove particularly useful in spectroscopy, especially ultra-high resolution spectroscopy that requires cold (slow), trapped (long interaction time) samples. Similar to cold atoms, cold molecules could be interrogated at a level of detail which is likely to provide new insights into their structure and interactions.

Trapping cold molecules

An experiment is described in which 108 CaH molecules have been confined in a magnetic trap at a temperature of 400 mK. The elastic scattering cross section of CaH on 3He was measured to be σelequals(1.5±0.6)×10-14cm2. A lower bound of Γrotational⩾10-15cm3s-1 was placed on the rotational elastic collision rate coefficient, and an upper bound of Γν<10-15 cm3s-1 was placed on the vibrational relaxation rate coefficient. The rate coefficient for spin changing collisions was measured to be, within an order magnitude, Γ Zequals10-17 cm3 s-1.