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. Evidently, the special beta-can structure of the protein provides shielding of the chromophore from the surroundings without significantly changing the electronic structure of the chromophore through interactions with amino acid side chains.

Erratum: Supplemental simulation to "Estimate of temperature and its uncertainty in small systems" [Am.

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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. Deviations from the 1/t law for such an ensemble give important information on the dynamics of the systems. As examples, we present measurements revealing strong radiative cooling of anions of both metal clusters and fullerenes.

We have studied the radiative cooling of negatively charged fullerene ions by following the thermionic emission as a function of time after injection into the heavy-ion storage ring ASTRID. It is argued that electron emission can be used as a calibrated thermometer to measure the cooling rate. For C 60 2 at ϳ1500 K the cooling corresponds to a radiation intensity of ϳ190 eV͞s, which is 2 orders of magnitude more than expected from infrared active vibrations. [S0031-9007(96)01602-X] The formation of fullerene molecules is a surprisingly commonplace phenomenon, accompanying, for example, soot formation when you burn a candle [1]. And yet, the physics and chemistry of the process are far from simple and are still incompletely understood. We address here the radiative cooling of hot C 60 molecules. In Ref.[1], Smalley argues that electronic transitions can hardly be important because of the large HOMO-LUMO gap (highest occupied to lowest unoccupied molecular orbital, about 1.7 eV). Instead, he suggests that the radiation is emitted by the infrared active vibrations [2].An estimate of the cooling rate at T ϳ 1800 K was obtained by Kolodney, Budrevich, and Tsipinyuk from observation of the depletion of thermal C 60 beams by fragmentation, and they conclude that the measured cooling is much faster than expected from emission by infrared active vibrations [3]. It is difficult to judge the accuracy of this result because it is derived from observation in a rather short time interval of the competition between cooling and fragmentation, C 60 ! C 58 1 C 2 , and the activation energy for this process is not known. We have studied the cooling of negatively charged fullerenes by observation over two decades in time of the competition between cooling and electron emission. The electron affinity is well known [1,4], and, with the additional information available on attachment cross sections for low-energy electrons [5-7], a reliable statistical formula can be established for thermionic electron emission. The formula can be tested against lifetimes measured for C 60 2 molecules with definite temperature [6], and hence thermionic emission can be used as a calibrated thermometer.The experiments were performed at the heavy-ion storage ring ASTRID [8]. A pulse of negatively charged fullerene ions from an electron-impact ion source was injected into the ring at 50 keV, and the decay of the stored current was followed by the observation of neutral decay products with a channel-plate detector in one of the four 90 ± magnets of the ring. As shown in Fig. 1, there is initially a high rate which is attributed to thermionic emission from the hot molecules. The rate decreases by 3-4 orders of magnitude until at t ϳ 100 ms it becomes so low that the yield of neutrals is dominated by collisions with rest-gas molecules, mainly H 2 . This contribution to the yield decays exponentially with a lifetime of order 10 s, corresponding roughly to a geometrical cross section for destruction.To interpret the data, we use a statistical model. The electro...

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Articles you may be interested inFirst storage of ion beams in the Double Electrostatic Ion-Ring Experiment: DESIREE Rev. Sci. Instrum. 84, 055115 (2013); 10.1063/1.4807702 Upgrade of the MIT Linear Electrostatic Ion Accelerator (LEIA) for nuclear diagnostics development for Omega, Z and the NIF Rev. Sci. Instrum. 83, 043502 (2012); 10.1063/1.3703315 The double electrostatic ion ring experiment: A unique cryogenic electrostatic storage ring for merged ion-beams studies Rev. Sci. Instrum. 82, 065112 (2011);An electrospray ion source has been coupled to an accelerator that injects ions into an electrostatic heavy-ion storage ring. Since the dc ion current produced by electrospray ionization is low (ϳ10 6 ions/s), ions are accumulated in a cylindrical ion trap filled with a helium buffer gas. The ions are collisionally damped in the buffer gas and confined to the central trap region by a rf field. Extraction from the trap occurs within a few microseconds and after acceleration through 22 kV, the ions of interest are selected by a magnet according to their mass to charge ratio. The ion bunch is subsequently injected into the ring. Both positive and negative ions have been stored, with masses ranging over 3 orders of magnitude (ϳ10 2 -10 4 Da). From a pickup signal in the ring, the number of ions in a bunch is estimated to be of the order of 10 3 -10 4 when the accumulation time is 0.1 s. Our first measurements show that we can store a sufficient number of ions to study the decay of metastable ions and to determine relative destruction cross sections. The technique could be useful to probe conformers differing only in size. Furthermore, our setup can be used for spectroscopic measurements of the ion-photon interaction such as the excitation of ͓Cytochrome cϩ17H] 17ϩ protein ions with 532 nm photons.

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