J. Reho

Photoinduced Chemical Dynamics of High-Spin Alkali Trimers

Nanometer-sized helium droplets, each containing about 104 helium atoms, were used as an inert substrate on which to form previously unobserved, spin-3/2 (quartet state) alkali trimers. Dispersed fluorescence measurements reveal that, upon electronic excitation, the quartet trimers undergo intersystem crossing to the doublet manifold, followed by dissociation of the doublet trimer into an atom and a covalently bound singlet dimer. As shown by this work, aggregates of spin-polarized alkali metals represent ideal species for the optical study of fundamental chemical dynamics processes including nonadiabatic spin conversion, change of bonding nature, and unimolecular dissociation.

Alkali–helium exciplex formation on the surface of helium nanodroplets. II. A time-resolved study

We have monitored the time evolution of the fluorescence of K*He exciplexes formed on the surface of helium nanodroplets using reversed time-correlated single photon counting. In modeling the present data and that from our previous work on Na*He, we find that partial spin–orbit coupling as well as the extraction energy of helium atoms from the droplet contribute to the observed dynamics of both K*He and Na*He formation, which differ considerably after either D1(n 2P1/2←n 2S1/2) or D2(n 2P3/2←n 2S1/2) excitation for both K(n=4) and Na(n=3). Our quantitative prediction of the Na*He formation dynamics coupled with preliminary data on and modeling of the formation dynamics of K*He allow for extrapolation to the case of Rb*He. Spin–orbit considerations combined with a simple model of helium atom extraction from the matrix reveal the following predicted trend: as the choice of the alkali guest atom is moved down the periodic table, alkali atom–He exciplex formation along the 1 2Π3/2 surface occurs faster while ...

Alkali–helium exciplex formation on the surface of helium nanodroplets. I. Dispersed emission spectroscopy

Dispersed emission spectra collected upon the 4 2P3/2,1/2←4 2S1/2 optical excitation of K atoms attached to helium nanodroplets include broad, structured, red-shifted features which are shown to be due to K*He exciplex formation, paralleling our former observation of Na*He [J. Reho, C. Callegari, J. Higgins, W. E. Ernst, K. K. Lehmann, and G. Scoles, Discuss. Faraday Soc. 108, 161 (1997)]. The exciplex formation is demonstrated by the agreement obtained in comparing the K*He A(1) 2Π→X(1) 2Σ emission spectra with the predictions derived from available ab initio potential energy surfaces. Recent analysis of both exciplex emissions also points to the possibility of triatomic (Na*He2 and K*He2) exciplex formation for a small fraction of the alkali atoms. The lack of fluorescence quenching, which is present when the spectra are taken in bulk liquid helium, is due to the surface location of the alkali atoms on the helium droplets that allows the nascent Na*He and K*He exciplexes to desorb from the droplet and e...

On the importance of exchange effects in three-body interactions: The lowest quartet state of Na3

Three-body interactions in a homonuclear van der Waals bound trimer (the 1 4A2′ state of Na3) are studied spectroscopically for the first time using laser induced emission spectroscopy on a liquid helium nanodroplet coupled with ab initio calculations. The van der Waals bound, spin polarized sodium trimers are prepared via pickup by, and selective survival in, a beam of helium clusters. Laser excitation from the 1 4A2′ to the 2 4E′ state, followed by dispersion of the fluorescence emission, allows for the resolution of the structure due to the vibrational levels of the lower state and for the gathering of precise information on the three-body interatomic potential. From previous experiments on Na2 we know that the presence of the liquid helium perturbs the spectra by a very small amount [see J. Higgins et al., J. Phys. Chem. 102, 4952 (1998)]. Ab initio potential energy calculations are carried out at 42 geometries of the lowest quartet state using the coupled cluster method at the single, double, and non...

Spectroscopy of Mg atoms solvated in helium nanodroplets

We have measured the laser-induced fluorescence excitation spectra of the 3 1P10←3 1S0 transition of Mg atoms solvated in helium nanodroplets. The observed blue shifts and line broadenings mirror the shifts and broadenings observed in studies of Mg atoms solvated in bulk liquid helium. This similarity allows us to conclude that Mg atoms reside in the interior of the helium droplet. The 3 1P10←3 1S0 transition shows a splitting which we attribute to a quadrupolelike deformation of the cavity which forms around the solute atom after excitation. Temporal evolution of the fluorescence from the solvated 3 1P10 Mg yields a longer lifetime (2.39±0.05 ns) than found in vacuum (1.99±0.08 ns). This difference can be accounted for quantitatively by evaluation of the anisotropic distribution of the helium density in the neighborhood of the excited Mg atom. The question of solvation vs surface location for the guest atoms is also discussed in light of the model of Ancilotto et al. [F. Ancilotto, P. B. Lerner, and M. W...

Photoinduced nonadiabatic dynamics in quartet Na3 and K3 formed using helium nanodroplet isolation

Helium nanodroplet isolation is used to produce van der Waals-bound quartet state alkali trimers (Na3 and K3) selectively over the corresponding chemically bound doublet trimers. Frequency-resolved excitation and emission spectroscopy reveals the presence of nonadiabatic spin–flip processes in the electronically excited states. A total of four quartet to quartet electronic transitions are observed: the 2 4E′,1 4E←1 4A2′ transitions of Na3 and the 1 4A1″,2 4E′←1 4A2′ transitions of K3. Time-resolved spectroscopy reveals that intersystem crossing times in Na3 decrease from 1.4 ns after excitation of the 0–0 band to approximately 400 ps for the higher vibronic levels (3,5/2). Analysis of the resonant quartet fluorescence reveals that the excited electronic state cools vibrationally on a time scale that is comparable to, but slower than, the intersystem crossing time.

Spin–orbit effects in the formation of the Na–He excimer on the surface of He clusters

In this paper we describe an application of reversed time-correlated single photon counting to the time-resolved spectroscopy of impurity atoms and molecules bound to large quantum clusters. The photo-induced dynamics of Na atoms on the surface of He and H2 clusters have been studied by following the time dependence of their emission at selected excitation and emission wavelengths. Collection of atomic (16980±145 cm-1) fluorescence arising from Na atoms excited on the cluster surface and immediately desorbed from it yields a finite (ca. 70 ps) rise time and a decay time of 16.3±0.1 ns, equal to the known lifetime of the 3P→3S transition of atomic Na. The frequency distribution of the red emission due to atoms that do not leave the cluster immediately after excitation is shown to be due to a desorbed Na*–He exciplex by obtaining quantitative agreement with predictions derived from available abinitio Na–He potentials. Formation of this excimer can occur along either the 2Π1/2 or 2Π3/2 excited state surface. ‘Slow’ (700 ps) and ‘fast’ (ca. 70 ps) components of the rise time of the red emission (15800±125 cm-1) are assigned, respectively, to the two formation channels. Introducing spin–orbit coupling effects into the long range abinitio pair potential for an isolated Na*–He generates a small barrier on the 2Π1/2 potential curve, which is linked to the observed slow exciplex formation time.

Spectroscopy in, on, and off a Beam of Superfluid Helium Nanodroplets

Helium nanodroplet isolation (HENDI) spectroscopy involves the use of a beam of He n (103 < n < 105) nanodroplets which are doped while passing, largely undeflected, through a pick-up cell containing the low pressure vapor of the substance to be examined. The nanodroplets carry the dopant species downstream, where they can be spectroscopically interrogated. After briefly reviewing the field and providing a few examples of applications, in this paper we show that HENDI is also useful in producing collimated beams of cold, gas-phase molecules which, after forming on the cold droplet’s surface, spontaneously desorb from it at very low velocities. As this low desorption velocity is added to the relatively large average velocity of the droplets, the desorbed molecules fly forward as part of the main beam and can be used to obtain sub-doppler high resolution spectra. Three examples of applications of this new technique will be reported here showing the spectrum of a well known molecule (ground state Na2), that of a less well known molecule (lowest triplet state Na2) and a third spectrum which is tentatively assigned to a yet unknown molecule: Na2 He.

Dynamics of the 13Πg state of K2 on helium nanodroplets

Fluorescence following optical excitation of the 13Σu+ state of K2 prepared on helium nanodroplets to the predissociative 13Πg state yields molecular emission from both the (B)11Πu and (A)11Σu+ K2 states as well as atomic emission from the expected 42P3/2,1/2 → 42S1/2 dissociation channel. A ∽12 cm−1 red shift is observed in the molecular emission excitation spectrum compared to the atomic emission excitation spectrum. Time-correlated photon counting measurements demonstrate the rise time for both atomic and molecular products to be <80 ps, independent of vibrational level excited. This lifetime is interpreted as the total depopulation time for the optically excited 13Πg state, which is dominated by intersystem crossing at low vibrational energy and by predissociation at the highest vibrational level. It is deduced that the timescale for intersystem crossing must be of the order of 10 ps. Symmetry restrictions for the isolated K2 imply that the intersystem crossing from the 13Πg state to the (B)11Πu and (A)1 1Σu+ states must be induced by interaction with the helium nanodroplet.