A mass and time-of-flight spectroscopy study of the formation of clusters in free-jet expansions of normal D2

Abstract

The mass spectra in the range of 2(D+)-38(D19+) amu of clusters formed in a supersonic free-jet expansion of normal D2 are investigated as functions of source temperature in the range of 95-220 K and of source pressure in the range of 10-120 bars. For some of the small ion fragments, time-of-flight distributions are also measured. For large clusters (n > 200) the intensities of the odd-numbered ion fragments exhibit magic numbers at D9+ and D15+ in accordance with previous experiments and calculations. The even-numbered ion fragments have much smaller intensities and exhibit new magic numbers at D10+ and D14+. For source conditions such that large clusters are formed, the intensities of the various different ion fragments are observed to saturate beyond a certain source pressure. At lower source pressures, where only small clusters are formed, the terminal mole fractions of the neutral dimers are analyzed in the light of available theories which take into account both the thermodynamics and the kinetics of the expansion. At higher source pressures and lower temperatures, where larger clusters are formed, the sizes of the neutral clusters are estimated using scaling laws and are found to be consistent with the mass spectra and measured time-of-flight distributions. By using a variety of techniques it has been possible to obtain reliable conclusions about the neutral cluster sizes for the present free-jet expansion conditions.

Adsorption of hydrogen on neutral and charged fullerene: Experiment and theory

“…According to an ab initio direct dynamics calculation, vertical ionization of the H 2 dimer, trimer, or hexamer leads to rapid formation of a vibrationally hot H 3 + and ejection of an energetic hydrogen atom even though H 3 + H is intrinsically stable with respect to H loss. 57 Electron ionization of hydrogen clusters embedded in helium nanodroplets (in the absence of C 60 ) also results in predominantly odd-numbered hydrogen cluster ions 44,58 even though the ionization mechanism is very different, namely, charge transfer from helium cations 15,59,60 versus direct ionization (details of the ionization mechanism will be discussed in Sec. VII B).…”

Formation of even-numbered hydrogen cluster cations in ultracold helium droplets

“…32 Another, more technical problem is the presence of deuterium; clusters H n−2 D + with an odd number of atoms may easily be mistaken for even-numbered H n + because both have the same nominal mass, n Da. An analogous problem applies to experiments with deuterium clusters contaminated with traces of 1 H. 27,28 In the present study we were able to use a highresolution mass spectrometer to identify even-numbered H n + up to n = 120. Two factors made this possible: First, the high mass resolution easily distinguishes H n + from most background ions, helium cluster ions, and mixed He x H y + .…”

“…20 When free hydrogen clusters are ionized by electrons or photons, one observes predominantly odd-numbered cluster ions H n + . [21][22][23][24][25][26][27][28] The low intensity of even-numbered H n + arises from the large exothermicity of the reaction H 2 + + H 2 → H 3 + + H, ͑1͒…”

Probing Phase Transitions with Supersonic Beams – A Joint Experimental and Theoretical Study

“…Since the very first generation of cluster beams under high vacuum conditions [66] it has been confirmed by a number of experiments that the mean flow velocity of supersonic beams may increase substantially with increasing stagnation pressure [17,[67][68][69][70][71]. Although these observations have been routinely ascribed to the formation of clusters in the expanding jet, the opposite behavior, a reduced flow velocity at higher source pressures, has been reported, too [9,35,42,69,71].…”