RESUMO
In this paper we report on the design and operation of a novel piezovalve for the production of short pulsed atomic or molecular beams. The high speed valve operates on the principle of a cantilever piezo. The only moving part, besides the cantilever piezo itself, is a very small O-ring that forms the vacuum seal. The valve can operate continuous (dc) and in pulsed mode with the same drive electronics. Pulsed operation has been tested at repetition frequencies up to 5 kHz. The static deflection of the cantilever, as mounted in the valve body, was measured as a function of driving field strength with a confocal microscope. The deflection and high speed dynamical response of the cantilever can be easily changed and optimized for a particular nozzle diameter or repetition rate by a simple adjustment of the free cantilever length. Pulsed molecular beams with a full width at half maximum pulse width as low as 7 micros have been measured at a position 10 cm downstream of the nozzle exit. This represents a gas pulse with a length of only 10 mm making it well matched to for instance experiments using laser beams. Such a short pulse with 6 bar backing pressure behind a 150 microm nozzle releases about 10(16) particles/pulse and the beam brightness was estimated to be 4x10(22) particles/(s str). The short pulses of the cantilever piezovalve result in a much reduced gas load in the vacuum system. We demonstrate operation of the pulsed valve with skimmer in a single vacuum chamber pumped by a 520 l/s turbomolecular pump maintaining a pressure of 5x10(-6) Torr, which is an excellent vacuum to have the strong and cold skimmed molecular beam interact with laser beams only 10 cm downstream of the nozzle to do velocity map slice imaging with a microchannel-plate imaging detector in a single chamber. The piezovalve produces cold and narrow (Delta v/v=2%-3%) velocity distributions of molecules seeded in helium or neon at modest backing pressures of only 6 bar. The low gas load of the cantilever valve makes it possible to design very compact single chamber molecular beam machines with high quality cold and intense supersonic beams. The high speed cantilever piezovalve may find broad applicability in experiments where short and strong gas pulses are needed with only modest pumping, the effective use of (expensive) samples, or the production of cold atomic and molecular beams.
RESUMO
In this paper we report on the in situ characterization of the cold velocity distribution of a pulsed molecular beam produced by a novel cantilever piezo valve. The velocity distribution is measured at various temporal positions within the pulsed expansion using femtosecond velocity map ion imaging. It is shown that the universal detection of molecules by multi-photon femtosecond velocity map ion imaging can provide directly the velocity distribution with excellent velocity resolution. The novel cantilever piezo valve can operate both in continuous (DC) and pulsed mode without any modification using the same drive electronics. Pulsed operation was tested at repetition rates of 20 Hz, 1 kHz and 5 kHz and a conical nozzle 200 mum in diameter. The cantilever valve produces a pulsed molecular beam of translationally cold molecules at modest backing pressures of about 6 bar. At low to medium repetition rates (20-1000 Hz) the pulsed piezo valve produces pulses of 12-40 mus duration of translationally cold seeded beams of helium and neon with speed ratios up to S = 135 (20 Hz, 0.1% CD(3)I in neon) and S = 55 (1 kHz). At the highest tested repetition rate of 5 kHz, the speed ratio obtained for the same seeded beam is reduced to about S = 45. This is still more than a factor of two better than the speed ratio S = 21 measured for a continuous beam produced with the same nozzle at 0.5 bar backing pressure. The cold velocity distribution of the pulsed beam expansion as compared to a continuous beam expansion is beneficial for improved spatial resolution in velocity map ion imaging experiments at high repetition rates of 1-5 kHz. The cantilever piezo valve has a simple design and may find broad applicability in areas where short gas pulses are warranted because of limited pumping speed, the effective use of (expensive) samples or the production of translationally and internally cold molecular beams at high repetition rate. When operating the piezo valve at high backing pressures of 15-30 bar extensive clustering of the low percentage (0.1-0.5%) seeded molecules in He and Ne carrier gasses can be produced of interest for cluster research.
