We study ionization and fragmentation of tetrafluoromethane (CF4) molecule induced by electron impact at low energies (E0 = 38 and 67 eV). We use a reaction microscope combined with a pulsed photoemission electron beam for our experimental investigation. The momentum vectors of the two outgoing electrons (energies E1, E2) and one fragment ion are detected in triple coincidence (e, 2e+ ion). After dissociation, the fragment products observed are CF3+, CF2+, CF+, F+ and C+. For CF3+ and CF2+ channels, we measure the ionized orbitals binding energies, the kinetic energy (KE) of the charged fragments and the two-dimensional (2D) correlation map between binding energy (BE) and KE of the fragments. From the BE and KE spectra, we conclude which molecular orbitals contribute to particular fragmentation channels of CF4. We also measure the total ionization cross section for the formation of CF3+ and CF2+ ions as function of projectile energy. We compare our results with earlier experiments and calculations for electron-impact and photoionization. The major contribution to CF3+ formation originates from ionization of the 4t2 orbital while CF2+ is mainly formed after 3t2 orbital ionization. We also observe a weak contribution of the (4a1)−1 state for the channel CF3+.

AbstractIn der Krebs‐Strahlentherapie werden halogenierte Nukleinbasen als Radiosensibilisatoren eingesetzt, um die Reaktivität der DNA gegenüber niederenergetischen Elektronen (NEEs) zu erhöhen. NEEs erzeugen DNA‐Strangbrüche bei spezifischen Energien (Resonanzen) durch dissoziative Elektronenanlagerung (DEA). Obwohl halogenierte Nukleinbasen intensive DEA‐Resonanzen bei verschiedenen Elektronenenergien in der Gasphase aufweisen, kann der Einfluss der halogenierten Nukleinbasen auf tatsächliche DNA‐Strangbrüche grundsätzlich nur schwer über den Energiebereich, in dem DEA stattfindet (<12 eV), untersucht werden. Mithilfe von DNA‐Origami‐Nanostrukturen haben wir die Energieabhängigkeit der Wirkungsquerschnitte für DNA‐Strangbrüche von Oligonukleotiden bestimmt, die mit 8‐Bromadenin (8BrA) modifiziert wurden. Diese Ergebnisse wurden mit DEA‐Messungen an isoliertem 8BrA in der Gasphase verglichen. Entgegen der Erwartungen wird der Großteil der Strangbrüche durch Resonanzen um 7 eV hervorgerufen, wohingegen der Einfluss von Resonanzen bei sehr niedrigen Energien (<2 eV) auf die Strangbrüche gering ist.

Halogenated nucleobases are used as radiosensitizers in cancer radiation therapy, enhancing the reactivity of DNA to secondary low-energy electrons (LEEs). LEEs induce DNA strand breaks at specific energies (resonances) by dissociative electron attachment (DEA). Although halogenated nucleobases show intense DEA resonances at various electron energies in the gas phase, it is inherently difficult to investigate the influence of halogenated nucleobases on the actual DNA strand breakage over the broad range of electron energies at which DEA can take place (<12 eV). By using DNA origami nanostructures, we determined the energy dependence of the strand break cross-section for oligonucleotides modified with 8-bromoadenine (8Br A). These results were evaluated against DEA measurements with isolated 8Br A in the gas phase. Contrary to expectations, the major contribution to strand breaks is from resonances at around 7 eV while resonances at very low energy (<2 eV) have little influence on strand breaks.

We have developed a multi target, Low Energy Electron (LEE), precise dose controlled irradiator for biomolecular films. Up to seven samples can be irradiated one after another at any preset electron energy and dose under UHV conditions without venting the chamber. In addition, one more sample goes through all the steps except irradiation, which can be used as control for comparison with the irradiated samples. All the samples are protected against stray electron irradiation by biasing them at -20 V during the entire period, except during irradiation. Ethernet based communication electronics hardware, LEE beam control electronics and computer interface were developed in house. The user Graphical User Interface to control the irradiation and dose measurement was developed using National Instruments Lab Windows CVI. The working and reliability of the dose controlled irradiator has been fully tested over the electron energy range of 0.5 to 500 eV by studying LEE induced single strand breaks to ΦX174 RF1 dsDNA.

We probe the interaction of myoglobin with intense, femtosecond laser pulses. Significant spectral differences are found between native and the irradiated myoglobin. These arise from the disruption of the heme prosthetic group: geometrical restructuring results in alteration of the oxidation state of Fe (from its initial +3 state) which is found to be reversible on timescales of ~4–6 h. Measurements taken upon addition of OH scavengers establish the key role played by these radicals in the overall dynamics. Myoglobin remains intact upon intense field irradiation, demonstrating the structural robustness of the polypeptide backbone. Experiments utilizing intense, ultrashort laser pulses are expected to open new horizons for following, with high sensitivity, changes in the oxidation state, chemical environment, and electronic state of biomolecules in the aqueous phase.

In this article we report the usage of (1) ΦX174 dsDNA as a model for electron – DNA interaction studies, (2) semiconductor grade 100 silicon wafer, gold on chrome on glass, and tantalum foil substrates, drying process and effect of temperature, on the DNA film formation and its stability, (3) stability of DNA films formed from DNA suspended in nano pure water and with additives like NaOH and TE buffer, and (4) effect of 0.001 mM NaOH and TE buffer (at pH 7.5) additives on DNA damage induced by 25 to 100 eV electrons. The results show that when tantalum foils are used as a substrate, it results in films, which have DNA distributed fairly uniformly and is also stable against strand breaks affected due to the stress of the drying. Electron irradiation of DNA suspended in TE buffer result in the formation of only relaxed form. When the DNA is suspended in 0.001 mM NaOH and irradiated similarly, linear form and cross links are also formed, in addition to relaxed form. This could be likely due to the secondary electrons interacting with Na+ ions that are bound to the DNA causing a second strand break in the opposite strand.

Low energy electrons (LEEs) are produced in copious amounts by the primary radiation used in radiation therapy. The damage caused to the DNA by these secondary electrons in the energy range 5-22 eV has been studied to understand their possible role in radiation induced damage. Electrons are irradiated on dried films of plasmid DNA (pQE30) and analysed using agarose gel electrophoresis. Single strand breaks (SSBs) induced by LEE to supercoiled plasmid DNA show resonance structures at 7, 12, and 15 eV for low doses and 6, 10, and ∼18 eV at saturation doses. The present measurements have an overall agreement with the literature that LEEs resonantly induce SSBs in DNA. Resonant peaks in the SSBs induced by LEEs at 7, 12, and 15 eV with the lowest employed dose in the current study are somewhat different from those reported earlier by two groups. The observed differences are perhaps related to the irradiation dose, conditions and the nature of DNA employed, which is further elaborated.

Structure in CF2+ cross section at 35.2eV is attributed to a possible inner valence shell excited resonance with a lifetime of 81 atto seconds.

A new design of a linear time of flight mass spectrometer (ToFMS) is implemented that gives nearly field-free interaction region without compromising on the mass resolution. The design addresses problems that would arise in a conventional Wiley-McLaren type of ToFMS: (i) field leakages into the charged particle-molecule interaction region from various components of the mass spectrometer, including that through the high transparency mesh used to obtain evenly distributed electric fields; (ii) complete collection and transportation of the ions produced in the interaction region to the detector, which is essential for high sensitivity and cross section measurements. This ToFMS works over a wide range of masses from H(+) to a few hundred Daltons and would be the most suitable for low energy charged particle-molecule interaction studies. Performance of the ToFMS has been tested by measuring the partial ionization cross sections for electron impact on CF(4).

An experimental and theoretical investigation of the dissociative states of SF4>2+ is presented. Various two- and three-body decays have been investigated and an attempt has been made to assign the energy levels and the geometry of the dication.

Anion formation by electron impact from CF4 has been studied experimentally in the range 0–30eV electron energy. F− and CF3− anions were observed while F2− ions, if any, were below the detection limit. The resonance peaks for F− and CF3− were measured to be 6.5 and 7.1eV, respectively. Only F− is produced via the ion pair formation channel and the yield varies monotonically with energy above the threshold.

High resolution laser induced fluorescence spectrum of jet-cooled SO(2) is recorded toward the blue side of the Clement's A-Band in the region of 314-319 nm. Time resolved fluorescence measurements have been carried out for all the prominent peaks in this region. Most of the peaks exhibited double exponential decay profiles. Some of the rovibronic bands exhibited quantum beats with strong quantum beats observed at 315.261 and 315.271 nm. This is the first observation of quantum beats in SO(2) in the absence of any external magnetic or electric fields. The decay profiles of the beating rovibronic bands were fitted using a four-level model by least-squares fitting method. The fitting shows that all the measured bands were double exponential with a similar first lifetime of approximately 3 mus and a varying second lifetime of the order of 1 micros-100 ns with a beating frequency of approximately 1 MHz. These quantum beats, in the absence of any external field, indicate rotational level mixing between the A (1)A(2) and the B (1)B(1) vibronic states which are near resonant due to the high density of states of these two states.

The influence of IR photon excitation of the ν3 vibrational mode in SF6 on dissociative electron attachment (SF5-formation) is investigated at high electron energy resolution (down to 1meV) over the energy range E=0–0.5eV with the laser photoelectron attachment method. The molecules are contained in a collimated seeded supersonic beam (nozzle temperatures T0=300–600K, corresponding to vibrational temperatures TV≈T0−100K) and transversely excited by the 10PX lines (X=10–40) in the 10.6μm band of a continuous CO2 laser at intensities up to about 400Wcm−2. The IR excitation and the attachment regions are separated by 5cm. The IR photon induced enhancement of the SF5- yield is found to be optimal on the 10P28 line (936.8cm−1) at all nozzle temperatures (with the maximum reached for T0≈390K) and monotonically decreasing with rising electron energy from 0eV over a range of about 0.3eV. For a fixed spatial profile of the exciting IR beam, the enhancement at E≈0eV follows a near-square-root dependence on laser power. With reference to previous work on the excitation of supersonic SF6 beams by CO2 laser light, the fraction of laser-excited SF6(ν3⩾1) molecules is estimated, and the absolute cross sections σL(E) for SF5- formation involving the IR-excited molecules are determined; they exceed the cross sections σ0(E) for thermal molecules in a way which strongly depends on electron energy and initial vibrational energy. In contrast, the cross section for SF6- formation is found to be almost independent of laser-excitation and temperature. The experimental findings are discussed with regard to the multiphoton character of the IR excitation, and comparisons are made with the effects of thermal excitation. The mechanisms for SF6-andSF5- formation and the responsible potential energy surfaces are discussed in the light of the available experimental data.

Fixed-nuclei R-matrix calculations for elastic electron-ozone scattering have been performed in the energy region between 0 and 10 eV. Electron correlation has been included extensively in the calculations. Applying the Fano-Feshbach projection operator formalism to the R-matrix results, we can extract from the calculations the positions and the energy-dependent widths of the resonances. The latter were used to estimate the magnitude of the total ion yield. We can show by comparison with experimental results that the low-lying (2)A(2) Feshbach resonance is very likely to be responsible for the observed pronounced maxima in the cross sections for the O- and the O-2(-) channels at 1.3 eV.

Vibrational excitation of the ν3 mode of SF6 by a CO2 laser, followed by electron attachment, leads to a very sharp enhancement in the dissociative electron attachment (DEA) process SF5−+F at 0 eV electron energy, but does not affect the intensity of the metastable SF6−. This is in striking contrast to thermal heating where SF5− is enhanced within a broad electron energy range and the SF6− intensity strongly decreases. In the framework of localized potential energy surfaces the effect of the ν3 mode for the DEA reaction at 0 eV is explained by the level of the curve crossing between the neutral and anionic system matching the energy of the ν3 vibration.

Photoion yields from gaseous fullerenes, C(60) and C(70), for production of singly and doubly charged ions are measured by mass spectrometry combined with tunable synchrotron radiation at hnu=25-150 eV. Since the signal of triply or highly charged ions is very weak, the total photoionization yield curve can be estimated from the sum of the yields of the singly and doubly charged ions. A distinct feature appears in the resultant curve of C(60) which is absent in the calculated total photoabsorption cross section previously reported. This difference is attributed to C(60) (2+) ions chiefly produced by spectator Auger ionization of the shape resonance states followed by tunneling of the trapped electron or by cascade Auger ionization. Ratios between the yields of doubly and singly charged ions for C(60) and C(70) are larger than unity at hnu>50 eV. These ratios are quite different from those reported in the experiments using electron impact ionization.

Total resonant cross sections for electron collision with ozone are presented. The results are based on ab initio calculations for the X(1)A(1) target ground state and the (2)A(2) negative-ion state potential energy surfaces close to the Franck-Condon region. The fixed-nuclei resonance width as a function of collision energy and internuclear distances has been obtained from R-matrix calculations. The nuclear motion has been taken into account employing the Franck-Condon reflection principle. In order to determine the information which is significant for the parametrization of resonant scattering cross sections, the calculations have been carried out at different levels of accuracy. The lifetime of the anionic state with respect to electron detachment was found to be long compared with the fragmentation process. Therefore, the fragmentation probability can be assumed to be equal to I and the total cross sections for the formation of the ion can be compared with the experimental cross sections for dissociative electron attachment (DEA). The results confirm the assumption that the prominent structure in the DEA cross sections at 1.3 eV is caused by a metastable (2)A(2) state of O-3(-).

A PC based data acquisition cum control system has been developed using a general purpose interface bus (GPIB/IEEE-488/IEC-625) plug in card and GPIB interfaceable NIM electronics for use in experiments on absorption spectroscopy studies of molecules using laser-induced fluorescence (LIF), resonance multiphoton ionization (REMPI), electron impact ionization and dissociative attachment from the ground state as well as from excited states, to name a few examples. Unlike commercially available multichannel scalers, this system can handle several parameters as the acquisition parameter (e.g., dye laser) is scanned over the desired wavelength in preselected segments saving precious data acquisition time. This system has been tested on laser excited dissociative electron attachment experiments.

Dissociative electron attachment (DEA) to vibronically excited SO2 in the Clements' band in the 288 to 298 nm region has been studied. The O- ion yield, to a first approximation, follows the photo absorption spectrum in this range indicating the DEA process to be independent of the vibrational levels in this band. This is in contrast to what is generally observed for vibrational level dependence from the electronic ground state. The current measurements also do not show any qualitative change in the dissociative attachment process due to change of symmetry as one moves from the peaks to the valleys of the Clements band.

Absolute cross sections for dissociative electron attachment (DEA) to NO 2 have been measured and compared with that of O 3 . A striking similarity is observed in the dissociation channels and the relative cross sections between the two molecules. This similarity is interpreted in terms of a common mechanism for electron capture to the two molecules and is exploited to characterize the nature of the NO 2 resonances in analogy with O 3 , using specific quantum chemical calculations as well as experimental data. This experimental observation and its interpretation seeks to direct attention to a previously uncommented on aspect of the DEA phenomenon in relation to molecules that are isoelectronically unrelated. The observation also tends to highlight the dominance of electronic polarizability in the electron-molecule scattering potential leading to capture.