We report doping of green fluorescent protein from an electrospray ionization (ESI) source into superfluid helium droplets. From analyses of the time profiles of the doped droplets, we identify two distinct groups of droplets. The faster group has a smaller average size, on the order of 106 helium atoms/droplet, and the slower group is much larger, by at least an order of magnitude. The relative populations of these two groups depend on the temperature of the droplet source: from 11 K to 5 K, the signal intensity of the slower droplet group gradually increases, from near the detection limit to comparable to that of the faster group. We postulate that the smaller droplets are formed via condensation of gaseous helium upon expansion from the pulsed valve, while the larger droplets develop from fragmentation of ejected liquid helium. Our results on the size and velocity of the condensation peak at higher source temperatures (> 7 K) agree with previous reports, but those at lower temperatures (< 7 K) seem to be off. We attribute this discrepancy to the masking effect of the exceedingly large droplets from the fragmentation peak in previous measurements of droplet sizes. Within the temperature range of our investigation, although the expansion condition changes from subcritical to supercritical, there is no abrupt change in either the velocity distribution or the size distribution of the condensation peak, and the most salient effect is in the increasing intensity of the fragmentation peak. The absolute doping efficiency, as expressed by the ratio of ion doped droplets over the total number of ions from the ESI source, is on the order of 10−4, while only hundreds of doped ions have been detected. Further improvements in the ESI source are key to extending the technology for future experiments. On the other hand, the separation of the two groups of droplets in velocity is beneficial for size selection of only the smaller droplets for future experiments of electron diffraction.

We report experimental observations of aniline (A) cations and He2+ when aniline is doped into ionized helium droplets. Large droplets containing 108 atoms are bombarded by energetic electrons, resulting in more than one positive charge in one droplet. When aniline encounters the charged droplets, some are ionized via charge transfer, while others can remain neutral in the presence of He2+ when the mass-to-charge ratio (m/z) of the droplet is sufficiently large. Upon resonant excitation of the dopant An or An+ (n ≥ 1), He2+ can be ejected. The excitation spectrum of He2+ becomes a juxtaposition of the spectra of An and An+. Moreover, an anticorrelation between the yields of He2+ and A+ is observed with increasing energies of the ionizing electrons. We attribute this result to the combined effect of reduction in m/z of the droplets and the different locations of He2+ and neutral An. Limited by the penetration depths of the ionizing electrons and further assisted by the Coulomb repulsion of coexisting cations, He2+ is located within 20 nm of the surface, while neutral An has an average position inside a large droplet. Upon resonant excitation of the interior An, He2+ is preferentially ejected. With increasing energies of the colliding electrons, the m/z of the droplets are reduced, leading to less effective charge shielding and more effective charge transfer, until ultimately, all He2+ can be neutralized to form A+.

No abstract

No abstract