In situ characterization of a cold and short pulsed molecular beam by femtosecond ion imaging

Abstract

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.

A short pulse (7 μs FWHM) and high repetition rate (dc-5kHz) cantilever piezovalve for pulsed atomic and molecular beams

“…We further improved the mechanical design of the valve as compared to our recently reported version. 20 At present we are able to produce seeded molecular beams in helium with pulses as short as FWHM= 7 s measured 10 cm downstream of the nozzle exit. In our improved design reported here no bouncing of the valve is observed even at repetition rates as high as 5 kHz.…”

“…Very recently, 20 we reported our first results using femtosecond velocity map ion imaging 15,16 to characterize in situ the speed distribution at various temporal positions within a short gas pulse produced by a novel cantilever piezovalve. Furthermore, we demonstrated the operation of the cantilever valve at repetition rates between dc and 5 kHz.…”

Toward elucidating the mechanism of femtosecond pulse shaping control in photodynamics of molecules by velocity map photoelectron and ion imaging

“…[35][36][37][38] In brief terms, the molecular beam was prepared by seeding 5% CH 2 BrCl in He which expands through a pulsed ͑1 kHz͒ homebuilt piezovalve with a 300 m nozzle. The molecular beam is doubly skimmed and crossed by femtosecond laser pulses about 150 mm downstream from the nozzle orifice.…”

Velocity map imaging of the dynamics of bimolecular chemical reactions

“…Signal levels can also be very low because of the need for seeded and skimmed molecular beams; improvements in the design of modern pulsed valves, giving shorter pulses of higher number density are having an impact here, however. 36,37 Generation of atom or radical beams of sufficient density is also a challenge, and usually requires a discharge or photolytic source. subsequently employed by Chandler, 41 Kitsopoulos, 31 Suits 42 and others.…”