Luminescence Spectroscopy of Rhodamine Homodimer Dications in Vacuo Reveals Strong Dye-Dye Interactions.

Being alone or together makes a difference for the photophysics of dyes but for ionic dyes it is difficult to quantify the interactions due to solvent screening and nearby counter ions. Gas-phase luminescence experiments are desirable and now possible based on recent developments in mass spectrometry. Here we present results on tailor-made rhodamine homodimers where two dye cations are separated by methylene linkers, (CH ) .

Photo excitation and laser detachment of C60(-) anions in a storage ring.

We have studied the photo physics of C60(-) anions in the electrostatic storage ring ELISA with ions produced in a plasma source and cooled and bunched in a He filled ion trap. A previous study using delayed electron detachment as a signal of resonance-enhanced multiphoton electron detachment (REMPED) has been repeated both at room temperature and with the trap cooled to liquid nitrogen temperature. However, wavelength dependence of the overlap of the strongly focused laser beam with the ion beam introduces distortions of the absorption spectrum.

Absolute cooling rates of freely decaying fullerenes.

The cooling rates of C60- have been measured in an electrostatic storage ring between several hundred mus and several tens of ms with one-photon laser excitation. The absolute energy scale is established by the photon energy, and the cooling time interval is derived from the nonexponential decay of the ensemble of hot molecules. The energy decreases due to the combined action of depletion and thermal emission of IR photons with a total energy loss rate that varies inversely proportional to time, 0.

Near-infrared photoabsorption by C60 dianions in a storage ring.

We present a detailed study of the electronic structure and the stability of C(60) dianions in the gas phase. Monoanions were extracted from a plasma source and converted to dianions by electron transfer in a Na vapor cell. The dianions were then stored in an electrostatic ring, and their near-infrared absorption spectrum was measured by observation of laser induced electron detachment.

Photodissociation of protonated tryptophan and alteration of dissociation pathways by complexation with crown ether.

The behavior of protonated tryptophan (TrpH(+)) and its complex with 18-crown-6-ether (CE) after photoexcitation has been explored based on measurements of dissociation lifetimes, fragmentation channels, and absorption spectra using an electrostatic ion storage ring. A recent implementation of pulsed power supplies for the ring elements with microsecond response times allows us to identify the daughter ion fragment masses and to disentangle fragmentation that occurs from excited states immediately after photoexcitation from that occurring on a longer time scale of several microseconds to milliseconds. We find that attachment of crown ether significantly alters the dissociation channels since it renders the pisigma(*)(NH(3)) state inaccessible and hence prevents the N-H bond breakage which is an important fragmentation channel of TrpH(+).

A new technique for time-resolved daughter ion mass spectrometry on the microsecond to millisecond time scale using an electrostatic ion storage ring.

A new method for time-resolved daughter ion mass spectrometry is presented, based on the electrostatic ion storage ring in Aarhus, ELISA. Ions with high internal energy, e.g.

Crystal blocking measurements of the time delay of fission induced by 32S, 48Ti, and 58Ni bombardment of W.

The time delay in fission induced by bombardment of W with 180 MeV 32S, 240-255 MeV 48Ti, and 315-375 MeV 58Ni has been measured by observation of crystal blocking. There is a clear narrowing and a small increase in the minimum yield of the angular dips for fission compared with scaled dips for elastically scattered ions. This is interpreted as a fission delay of about 2 as, only weakly dependent on energy and atomic number.

Dianions of 7,7,8,8-tetracyano-p-quinodimethane and perfluorinated tetracyanoquinodimethane: information on excited states from lifetime measurements in an electrostatic storage ring and optical absorption spectroscopy.

We have developed an experimental technique that allows us to study the physics of short lived molecular dianions in the gas phase. It is based on the formation of monoanions via electrospray ionization, acceleration of these ions to keV energies, and subsequent electron capture in a sodium vapor cell. The dianions are stored in an electrostatic ion storage ring in which they circulate with revolution times on the order of 100 micros.

Lifetimes of C60(2-) and C70(2-) dianions in a storage ring.

C60(2-) and C70(2-) dianions have been produced by electrospray of the monoanions and subsequent electron pickup in a Na vapor cell. The dianions were stored in an electrostatic ring and their decay by electron emission was measured up to 1 s after injection. While C70(2-) ions are stable on this time scale, except for a small fraction of the ions which have been excited by gas collisions, most of the C60(2-) ions decay on a millisecond time scale, with a lifetime depending strongly on their internal temperature.

Nonthermal power law decay of metal dimer anions.

The metastable decay of dimer anions of Cu and Ag has been measured in a storage ring. The decay is found to proceed nonexponentially and is well described by a power law with an exponent of -1. This signals the presence of a continuum of decay constants in the ensemble.

Dynamic Jahn-Teller effects in isolated C60- studied by near-infrared spectroscopy in a storage ring.

We have measured the near-infrared absorption spectrum for isolated C60- ions at room temperature. Two bands, at 9145 cm(-1) and 10460 cm(-1), have been identified in addition to the main absorption band at 9382 cm(-1), seen also at low temperature in a matrix. An interpretation based on the theory of dynamic Jahn-Teller effects is proposed.

Photophysics of protoporphyrin ions in vacuo: triplet-state lifetimes and quantum yields.

Lifetimes of triplet-state molecules and triplet quantum yields are important parameters in photobiology as they determine the generation of singlet-oxygen upon irradiation with visible light. Here we report lifetimes of protoporphyrin IX (pp) in vacuo measured in an ion storage ring. We find that after 532 nm photon absorption, pp(-) (free base and negatively charged carboxylate) and pp(+) (single protonation of ring nitrogen) have triplet-state lifetimes of 12 and 6 ms, respectively.

Photodestruction of adenosine 5'-monophosphate (AMP) nucleotide ions in vacuo: statistical versus nonstatistical processes.

The lifetimes of both deprotonated adenosine 5(')-monophosphate (AMP) and protonated AMP, after 266-nm photon absorption and intramolecular vibrational redistribution, were determined to be 16 micros based on a newly developed heavy-ion storage ring technique. Protonation of the adenine nucleobase seriously affects the fragmentation mechanism: the major part of the photoexcited anions fragment in a statistical (ergodic) process, whereas nearly all the cations (>99%) fragment in a nonergodic process. In solution at natural pH the AMP anion is prevalent and photoproduct formation is therefore less important as the time for vibrational cooling is of the order of picoseconds.

Absorption spectrum of the green fluorescent protein chromophore anion in vacuo.

A sensitive photoabsorption technique for studies of gas-phase biomolecules has been used at the ELISA electrostatic heavy-ion storage ring. We show that the anion form of the chromophore of the green fluorescent protein in vacuo has an absorption maximum at 479 nm, which coincides with one of the two absorption peaks of the protein. Its absorption characteristics are therefore ascribed to intrinsic chemical properties of the chromophore.

Observation of a 1/t decay law for hot clusters and molecules in a storage ring.

The exponential law is valid both for decay from a single quantum state into a continuum and for an ensemble maintained in thermal equilibrium. For statistical decay of an ensemble of isolated systems with a broad energy distribution, the exponential decay is replaced by a 1/t distribution. We present confirmation of this decay law by experiments with cluster anions in a small electrostatic storage ring.

Electron loss from multiply protonated lysozyme ions in high energy collisions with molecular oxygen.

We report on the electron loss from multiply protonated lysozyme ions Lys-Hn(n)+ (n = 7 - 17) and the concomitant formation of Lys-Hn(n+1)+. in high-energy collisions with molecular oxygen (laboratory kinetic energy = 50 x n keV). The cross section for electron loss increases with the charge state of the precursor from n = 7 to n = 11 and then remains constant when n increases further.

Dissociation energy for C(2) loss from fullerene cations in a storage ring.

We have stored positively charged fullerene ions C(+)(n) (n even, from 48 to 70 and 76), C(2+)(60) and C(2+)(70) in an electrostatic storage ring and have measured the rate of emission of neutral fragments as a function of time. In the time range of the measurements, 50 micros to a few milliseconds, the rate decreases strongly due to radiative cooling of the molecules. Using the cooling rate predicted from a dielectric model, we have extracted the dissociation energies for C(2) loss from the measurements.