Energy Transfer Distribution Research Articles

Is Constant RBE Accurate Enough for Determination of Biological Effects in Proton Therapy?

The relative biological effectiveness (RBE) of protons currently used in clinical practice is assigned a constant value of 1.1, despite the evidence that the actual RBE of protons varies as a function of proton energy, linear energy transfer (LET), dose per treatment fraction, tissue type, and clinical endpoint. The purpose of this study is to compare the performance of the traditional, constant RBE model, and two variable RBE models in explaining the radiation induced brain necrosis indicated on follow-up imaging studies for a patient treated with postoperative proton radiation therapy for malignant meningioma. In this study, treatment plan information is extracted from a clinical treatment planning system (TPS) and used to recalculate physical proton dose and linear energy transfer (LET) distributions using Monte Carlo simulations. One constant and two variable RBE models which account for tissue specific parameters, LET, biological endpoint, and dose per treatment fraction are applied on a voxel-by-voxel basis to compute three biologically effective dose distributions. These dose distributions are subsequently correlated to follow-up imaging studies which have indicated the presence of radiation induced necrosis in the treated region of the patient being analyzed. Corresponding areas of toxicity and dose/RBE computed with three different models are compared to determine which model performs the best in explaining the radiation induced toxicity. Areas of increased biologically effective dose computed using variable RBE models compared to the original dose distribution calculated by the clinical TPS using a constant RBE model are found to correspond to areas of radiation induced brain necrosis. Variable RBE doses were up to 8% higher in the region of toxicity. Variable RBE models perform better in explaining the radiation induced brain necrosis.

SU-E-T-552: Maximizing the Biological Effect of Proton Dose Delivered with Scanned Beam.

Biological effect of radiation can be enhanced with hypofractionation, localized dose escalation, and controlled distribution of proton's linear energy transfer (LET). We evaluate potential gain in therapeutic effect from delivery of daily inhomogeneous fractional dose distributions in pencil beam scanning proton therapy (PBS-PT). For cases of prostate cancer, we considered a hypofractionated course of 20 fractions of 3 Gy (assuming α/β=1.5, the equivalent dose in 2-Gy fractions (ED2Gy) is 77.1 Gy). Two sets of dose distributions were planned using two opposed lateral fields to deliver a uniform dose: (1) in full-target plans (FTP) each beam targeted the entire gland (2) in split-target plans (STP), beams targeted only the respective proximal hemispheres (prostate split sagittally). Linear combinations of optimized beam intensity maps from FTP and STP, for a variety of mixing weights, were used to evaluate inhomogeneous fractional dose (IFD) distributions. IFD delivered doses boosting either hemisphere in alternating fractions, e.g., alternating between 40% and 160% of the nominal fractional dose (1.2-4.8 Gy). The equivalent uniform dose (EUD) was calculated for ED2Gy distributions. IFD plans were rescaled so that the EUD of rectum and bladder did not increase. LET distributions were calculated with Monte Carlo, and compared for different plans. In the IFD courses, the whole prostate received a nearly uniform dose in every 2 fractions, however EUD was higher than in conventional FTP by up to 8%. Rectal EUD decreased by 2%, and bladder EUD was unchanged. The LET distributions of FTP and STP were distinctly different, thus, in IFD, LET depended strongly on the mixing weights. In PBS-PT, modestly improved therapeutic outcome can be expected with delivery of inhomogeneous daily dose distributions, while administering the prescribed dose to target over the entire course. The biological effectiveness may be further enhanced by optimizing the LET distributions. The project was supported by the Federal Share of program income earned by Massachusetts General Hospital on C06 CA059267, Proton Therapy Research and Treatment Center.

TH‐C‐213AB‐05: Correlating Areas of Radiation Toxicity with Relative Biological Effectiveness‐Weighted Dose Distributions in Proton Radiotherapy

Purpose: To determine if biologically effective proton dose calculated using a relative biological effectiveness (RBE) model with multiple parameters may be used to explain areas of radiation induced brain necrosis indicated on follow‐up imaging studies for a patient treated with postoperative intensity modulated proton radiotherapy for malignant meningioma. Methods: In this study, treatment plan information is extracted from a clinical treatment planning system (TPS) and used to recalculate physical proton dose and linear energy transfer (LET) distributions using Monte Carlo simulations. An RBE model which accounts for tissue specific parameters, LET, biological endpoint, and dose per treatment fraction is applied on a voxel‐by‐voxel basis to compute a biological effective dose distribution. This dose distribution is subsequently compared to follow‐up imaging studies which have indicated the presence of radiation induced necrosis in the treated region of the patient being analyzed. Corresponding areas of toxicity and dose/RBE are compared to determine the governing relationship. Results: Areas of increased biological effective dose compared to the original dose distribution calculated by the clinical TPS are found to correspond to areas of radiation induced necrosis identified on follow‐up imaging. Biologically effective dose is found to be increased over TPS dose by as much as 8% of the prescribed dose for the patient. Conclusions: Dose distributions which account for the variable nature of proton RBE may provide insight into toxicity risk for patients treated with proton radiotherapy.

SU-E-T-60: Development and Validation of a TOPAS Model of a Spot Scanning Proton Therapy Nozzle

Purpose: To create a Monte Carlo model of a commercial spot scanning proton therapy nozzle and validate pencil beam dose distributions simulatedusing the model. Methods: We used TOPAS (Tool for Particle Simulation), a Monte Carlo simulation tool based on the Geant4 Toolkit, in our simulations. In TOPAS, we implemented the geometry and material composition of the nozzle, based on information found in the literature (Gillin et al. 2010, Med. Phys. 37:154; Sawakuchi et al. 2010, Med. Phys. 37:4960). The following components were modeled: vacuum window, profile monitor (multi-wire ionization chamber), helium chamber, main and sub dose monitors (parallel plate ionization chambers), spot position monitor (multi-wire ionization chamber) and snout. We scored dose to medium using the standard scorers in TOPAS. No variance reduction techniques were used. Simulated dose distributions for several energies were compared to ionization chamber measurements. Source parameters, including size, angular spread and energy spread, were taken to be free parameters due tothe large uncertainties in the literature-provided data. Results: For energies ranging from 72.5 to 206.3 MeV, simulated pencil beam ranges agreed within 2.2 mm with experimental data. Simulated full width at half-maximum values for in-air lateral profiles agreed within the experimental uncertainties (∼0.5 mm) for three validated energies (72.5, 173.7 and 221.8 MeV). Conclusions: A spot scanning proton therapy nozzle has been modeled using TOPAS. Further work is underway to improve the agreement between simulated and experimental data of ranges and lateral profiles through adjustments of the source parameters and physics list. Because TOPAS handles CT datasets, once validated, the model developed in this work may be used to perform retrospective studies of the impact of dose and linear energy transfer distributions in patient outcome. This model may also be used to investigate the response of radiation detectors exposed to scanning proton beams. Natural Sciences and Engineering Research Council of Canada and Ontario Graduate Scholarship Program, Ontario Ministry of Training, Colleges and Universities

A comparison of Ar ion implantation and swift heavy Xe ion irradiation effects on immiscible AlN/TiN multilayered nanostructures

We have compared the effects of 200keV 40Ar1+ ion implantation and 166MeV 132Xe27+ ion irradiation on immiscible (AlN/TiN)×5 multilayers grown on Si(100) wafers. The layers were deposited by reactive sputtering, individual layer thickness was ∼22nm (AlN) and ∼32nm (TiN), the stoichiometry Al:N∼45:55 and Ti:N∼50:50at%. Argon was implanted to 4×1016ionscm−2, and xenon to 5×1014ionscm−2. The projected Ar range is around mid depth of the multilayered structure, while swift Xe ions are buried deep into the Si substrate. Upon irradiation the structures remain essentially stable and unmixed; although in both cases we observed detectable effects. The use of wide range of irradiation parameters (Se/Sn=1.2–1.4, dpa=42–63 for Ar; and Se/Sn=249–258, dpa=0.03–0.05 for Xe) enabled to distinguish between the contribution of nuclear and electronic stopping. In case of Ar implantation both atomic collisions and electronic excitations contribute to the induced structural modifications, and in case of Xe only electronic excitations. It was deduced that electronic excitations generate local heating which influences lateral grain growth within individual layers, but no elemental redistribution. On the other hand, atomic collisions facilitate a low level of Ti migration into the under-stoichiometric AlN layers, in the vicinity of the implanted Ar ion range. Energy transfer and temperature distribution were evaluated and compared to the effects produced in the structures. The presented results can be interesting towards developing radiation tolerant materials.

Dose and dose averaged LET comparison of 1H, 4He, 6Li, 8Be, 10B, 12C, 14N, and 16O ion beams forming a spread‐out Bragg peak

Purpose:Modern clinical accelerators are capable of producing ion beams from protons up to neon. This work compares the depth dose distribution and corresponding dose averaged linear energy transfer (LET) distribution, which is related to the biological effectiveness, for different ion beams (1H, 4He, 6Li, 8Be, 10B, 12C, 14N, and 16O) using multi‐energetic spectra in order to configure spread‐out Bragg peaks (SOBP).Methods:Monte Carlo simulations were performed in order to configure a 5 cm SOBP at 8 cm depth in water for all the different ion beams. Physical dose and dose averaged LET distributions as a function of depth were then calculated and compared. The superposition of dose distribution of all ions is also presented for a two opposing fields configuration. Additional simulations were performed for12C beams to investigate the dependence of dose and dose averaged LET distributions on target depth and size, as well as beam configuration. These included simulations for a 3 cm SOBP at 7, 10, and 13 cm depth in water, a 6 cm SOBP at 7 depth in water, and two opposing fields of 6 cm SOBP.Results:Alpha particles and protons present superior physical depth dose distributions relative to the rest of the beams studied. Dose averaged LET distributions results suggest higher biological effectiveness in the target volume for carbon, nitrogen and oxygen ions. This is coupled, however, with relatively high LET values—especially for the last two ion species—outside the SOBP where healthy tissue would be located. Dose averaged LET distributions for8Be and 10B beams show that they could be attractive alternatives to 12C for the treatment of small, not deeply seated lesions. The potential therapeutic effect of different ion beams studied in this work depends on target volume and position, as well as the number of beams used.Conclusions:The optimization of beam modality for specific tumor cites remains an open question that warrants further investigation and clinically relevant results.

Alkylation Effects on the Energy Transfer of Highly Vibrationally Excited Naphthalene

The energy transfer of highly vibrationally excited isomers of dimethylnaphthalene and 2-ethylnaphthalene in collisions with krypton were investigated using crossed molecular beam/time-of-flight mass spectrometer/time-sliced velocity map ion imaging techniques at a collision energy of approximately 300 cm(-1). Angular-resolved energy-transfer distribution functions were obtained directly from the images of inelastic scattering. The results show that alkyl-substituted naphthalenes transfer more vibrational energy to translational energy than unsubstituted naphthalene. Alkylation enhances the V→T energy transfer in the range -ΔE(d)=-100~-1500 cm(-1) by approximately a factor of 2. However, the maximum values of V→T energy transfer for alkyl-substituted naphthalenes are about 1500~2000 cm(-1), which is similar to that of naphthalene. The lack of rotation-like wide-angle motion of the aromatic ring and no enhancement in very large V→T energy transfer, like supercollisions, indicates that very large V→T energy transfer requires special vibrational motions. This transfer cannot be achieved by the low-frequency vibrational motions of alkyl groups.

WE‐E‐BRB‐01: Voxelized Quenching Correction for Proton Pencil Beams in a Liquid Scintillator Detector

Purpose: To correct for quenching on a voxel‐by‐voxel basis for proton pencil beams in a liquid scintillation dosimetry system. Methods: Scintillation light from a proton pencil beam in a tank of organic liquid scintillator (20×20×20 cm3) was measured with a CCD camera for three beam energies (100.9, 144.9, and 161.6 MeV). Three‐dimensional dose and linear energy transfer (LET) distributions were calculated for each beam energy using a validated Monte Carlo model of the proton pencil beam. The quenching for each voxel in the scintillator was calculated using the LET and material‐specific empirical factors. The empirical factors for the scintillator were obtained by a fit to the Monte Carlo LET and dose data and the intensity values measured with the CCD camera for the central energy (144.9 MeV). The empirical values and Monte Carlo calculations were then used to predict the quenched light signal for the other two proton energies. The differences between the predicted and measured light signals were quantified using a gamma index of 3% or 3mm to agreement. Results: The LET in the distal falloff region was a factor of 10 higher than in the proximal build‐up region, while the measured light peak was 75% of the Bragg peak height. The calculated light signal derived from the voxelized quenching factors provided a good approximation of the measured light signal. Conclusion: The results of this study indicate that it is possible to correct for scintillator quenching on a voxel‐by‐voxel basis in a large detection volume. This correction requires a prior knowledge of the three‐dimensional LET distribution of the beam and the material‐dependent empirical quenching factors for the scintillator. This quenching correction method will be employed in future studies using liquid scintillators to obtain three‐dimensional dosimetric information from proton pencil beams.

Numerical study of the generation of linear energy transfer spectra for space radiation applications

In analyzing charged particle spectra in space due to galactic cosmic rays (GCR) and solar particle events (SPE), the conversion of particle energy spectra into linear energy transfer (LET) distributions is a convenient guide in assessing biologically significant components of these spectra. The mapping of LET to energy is triple valued and can be defined only on open energy subintervals where the derivative of LET with respect to energy is not zero. Presented here is a well-defined numerical procedure which allows for the generation of LET spectra on the open energy subintervals where, in spite of their singular nature, the spectra are integrable. The efficiency of the numerical procedures is demonstrated by providing examples of computed differential and integral LET spectra and their equilibrium components for historically large SPEs and 1977 solar minimum GCR environments. Due to the biological significance of tissue, all simulations are done with tissue as the target material.

Variations in Linear Energy Transfer Within Clinical Proton Therapy Fields and the Potential for Biological Treatment Planning

To calculate the linear energy transfer (LET) distributions in patients undergoing proton therapy. These distributions can be used to identify areas of elevated or diminished biological effect. The location of such areas might be influenced in intensity-modulated proton therapy (IMPT) optimization. Because Monte Carlo studies to investigate the LET distribution in patients have not been undertaken so far, the code is first validated with simulations in water. The code was used in five patients, for each of them three planning and delivery techniques were simulated: passive scattering, three-dimensional modulation IMPT (3D-IMPT), and distal edge tracking IMPT (DET-IMPT). The inclusion of secondary particles led to significant differences compared with analytical techniques. In addition, passive scattering and 3D-IMPT led to largely comparable LET distributions, whereas the DET-IMPT plans resulted in considerably increased LET values in normal tissues and critical structures. In the brainstem, dose-averaged LET values exceeding 5 keV/μm were observed in areas with significant dose (>70% of prescribed dose). In noncritical normal tissues, even values >8 keV/μm occurred. This work demonstrates that active scanning offers the possibility of influencing the distribution of dose-averaged LET (i.e., the biological effect) without significantly altering the distribution of physical dose. On the basis of this finding, we propose a method to alter deliberately the LET distribution of a treatment plan in such a manner that the LET is maximized within certain target areas and minimized in normal tissues, while maintaining the prescribed target dose and dose constraints for organs at risk.

MO-E-BRB-02: Biologically Optimized Treatment Planning for Proton Therapy: Monte Carlo Calculated Linear Energy Transfer Distributions in Patients

Purpose: To apply Monte Carlo simulations to simulate the Linear Energy Transfer (LET) distributions in patients undergoing proton therapy. These distributions can be used to identify areas of elevated biological effect. The location of such areas might be influenced in intensity‐modulated proton therapy (IMPT) optimization. Method and Materials:ProtonMonte Carlo calculations are performed using a code considering all relevant primary and secondary particles. Since Monte Carlo studies to investigate the LET distribution in patients have not been undertaken so far, the code is first validated: simulations in a water phantom are compared to published data. The code was subsequently used to track particles through the patient geometry based on CT information for five patients. For each of them three different proton therapy planning and delivery techniques were simulated: passive scattering, 3D modulation IMPT (3D‐IMPT) and Distal Edge Tracking IMPT (DET‐IMPT). Results: Detailed examination of the LET distributions and LET‐Volume‐Histograms reveals significant differences between the treatment techniques. While passive scattering and 3D‐IMPT lead to largely comparable LET distributions, the DET‐IMPT plans result in considerably increased LET values in normal tissues and organs at risk. In the brainstem, dose‐averaged LET values exceeding 10 keV/ m are observed in areas with significant dose levels (above 70% of prescribed dose). In non‐critical normal tissues even values above 15 keV/ m occur, although in areas with low doses. Conclusion: This work demonstrates that active scanning offers the possibility to influence the distribution of dose averaged LET (i.e. the biological effect) without significantly altering the distribution of physical dose. Based on this finding, we propose a method to deliberately alter the LET distribution of a treatment plan in such a manner that the LET is maximized within the target and minimized in normal tissues, while leaving the prescribed dose to the target unchanged.

Measurement of LET distribution and absorbed dose from secondary particles on board the spacecraft

Experimental technique for the measurement of linear energy transfer (LET) distribution and absorbed dose from the secondary particles of space radiation is considered. High sensitive nuclear emulsion detector is used for identification of nuclear fragments produced in nuclear interactions and recoil protons from elastic (n,p) scattering. The contribution of secondaries of different origin to the total LET spectrum is estimated.

Pathways of Energy Flow in LHCII from Two-Dimensional Electronic Spectroscopy

Photosynthetic light-harvesting complexes absorb energy and guide photoexcitations to reaction centers with speed and efficacy that produce near-perfect efficiency. Light harvesting complex II (LHCII) is the most abundant light-harvesting complex and is responsible for absorbing the majority of light energy in plants. We apply two-dimensional electronic spectroscopy to examine energy flow in LHCII. This technique allows for direct mapping of excitation energy pathways as a function of absorption and emission wavelength. The experimental and theoretical results reveal that excitation energy transfers through the complex on three time scales: previously unobserved sub-100 fs relaxation through spatially overlapping states, several hundred femtosecond transfer between nearby chlorophylls, and picosecond energy transfer steps between layers of pigments. All energy is observed to collect into the energetically lowest and most delocalized states, which serve as exit sites. We examine the angular distribution of optimal energy transfer produced by this delocalized electronic structure and discuss how it facilitates the exit step in which the energy moves from LHCII to other complexes toward the reaction center.

Multisite Structure of PbWO4:Eu3+ Crystals Investigated by Site-Selective Laser-Excitation Spectroscopy

Defect structures of Eu(3+)-doped PbWO(4) crystals are investigated by site-selective laser-excitation spectroscopy. The excitation spectrum of the (7)F(0) --> (5)D(0) transition shows a strong dependence on Eu(3+)-concentration in PbWO(4) crystals. Only one substitution site for Eu(3+) is identified in the PbWO(4):Eu(3+) (0.01 at.%) crystal, while at least six inequivalent sites are observed in the PbWO(4):Eu(3+) (0.05 and 0.5 at.%) crystals. The charge compensation mechanisms for the observed sites are discussed in relation with annealing effects and temperature-dependent decay curves. The dominant site for Eu(3+) in the PbWO(4) lattice seems to be due to a formation of Eu(Pb)(3+)-V(Pb)'' dipole by charge compensation. The minor sites for Eu(3+) are attributable to the Eu(Pb)(3+)-O(i)'' dipole, a site perturbed by Eu(Pb)(3+*)-V(Pb)'' dipole, and a clustering of Eu(3+) in the PbWO(4) lattice. The energy transfer and site distribution of Eu(3+) are discussed in PbWO(4):Eu(3+) (0.01, 0.05, and 0.5 at.%).

Effects of Alkylation on Deviations from Lennard−Jones Collision Rates for Highly Excited Aromatic Molecules: Collisions of Methylated Pyridines with HOD

Collision rates and energy transfer distributions are reported for HOD with highly vibrationally excited 2-methylpyridine (2-picoline, E = 38 310 cm(-1)) and 2,6-dimethylpyridine (2,6-lutidine, E = 38 700 cm(-1)). High resolution transient IR absorption is used measured to complete product state distributions of scattered HOD(000) molecules with E(rot) = 109 to 1331 cm(-1). Doppler-broadened line profiles characterize the depletion and appearance for HOD molecules due to collisions with hot donors and show that the product translational and rotational energy distributions are similar for both donors with DeltaE(rel) = 370 cm(-1) and DeltaE(rot) approximately 75 cm(-1). The energy transfer rate for picoline (E)/HOD is 2.5 times larger than the Lennard-Jones collision rate. The energy transfer rate for lutidine(E)/HOD is 3.2 times larger than the Lennard-Jones collision rate. Previous work ( Havey, Liu, Li, Elioff, and Mullin, J. Phys. Chem. A 2007, 111, 13321-9 ) reported similar energy transfer values for pyrazine/HOD collisions and an energy transfer rate that is 1.7 times the Lennard-Jones collision rate. The observed collision rates are discussed in terms of hydrogen bonding interactions between HOD and the aromatic donor molecules. Energy gain profiles for HOD are compared with those for H(2)O.

Calculation of energy-deposition distributions and microdosimetric estimation of the biological effect of a 9C beam

Among the alternative beams being recently considered for external cancer radiotherapy, (9)C has received some attention because it is expected that its biological effectiveness could be boosted by the beta-delayed emission of two alpha particles and a proton that takes place at the ion-stopping site. Experiments have been performed to characterise this exotic beam physically and models have been developed to estimate quantitatively its biological effect. Here, the particle and heavy-ion transport code system ( PHITS ) is used to calculate energy-deposition and linear energy transfer distributions for a (9)C beam in water and the results are compared with published data. Although PHITS fails to reproduce some of the features of the distributions, it suggests that the decay of (9)C contributes negligibly to the energy-deposition distributions, thus contradicting the previous interpretation of the measured data. We have also performed a microdosimetric calculation to estimate the biological effect of the decay, which was found to be negligible; previous microdosimetric Monte-Carlo calculations were found to be incorrect. An analytical argument, of geometrical nature, confirms this conclusion and gives a theoretical upper bound on the additional biological effectiveness of the decay. However, no explanation can be offered at present for the observed difference in the biological effectiveness between (9)C and (12)C; the reproducibility of this surprising result will be verified in coming experiments.

Probing complexes with single fluorophores: factors contributing to dispersion of FRET in DNA/RNA duplexes

Single molecule fluorescent microscopy is a method for the analysis of the dynamics of biological macromolecules by detecting the fluorescence signal produced by fluorophores associated with the macromolecule. Two fluorophores located in a close proximity may result in Förster resonance energy transfer (FRET), which can be detected at the single molecule level and the efficiency of energy transfer calculated. In most cases, the experimentally observed distribution of FRET efficiency exhibits a significant width corresponding to 0.07-0.2 (on a scale of 0-1). Here, we present a general approach describing the analysis of experimental data for a DNA/RNA duplex. We have found that for a 15 bp duplex with Cy3 and Cy5 fluorophores attached to the opposite ends of the helix, the width of the energy transfer distribution is mainly determined by the photon shot noise and the orientation factor, whereas the variation of inter-dye distances plays a minor role.

Measurement of a Linear Energy Transfer Distribution with Antioxidant Doped CR-39 Correcting for the Dip Angle Dependence of Track Formation Sensitivity

Antioxidant doped CR-39 detectors were loaded onto the STS-95 space shuttle mission (altitude: 574 km; inclination: 28.45°; flight duration: 8.9 days) for measuring the linear energy transfer (LET) distribution above 10 keV/µm for space radiation dosimetry. It is known that the track formation sensitivity of antioxidant doped CR-39 detectors depends on the dip angle of the incident particle. We investigated this dip angle dependence for a wide range of LET values and dip angles. The track formation sensitivities at lower dip angles were obviously decreased in the LET region below 100 keV/µm. We introduced minimum-cutoff dip angles in order to correct for such dip-angle dependence. The LET distribution of the STS-95 mission was obtained from the measurements of etch pits having dip angles larger than the minimum-cutoff dip angles. This new correction method increased the absorbed dose and dose equivalent above 10 keV/µm by 54 and 28%, respectively.

Sensitivity of ion-induced sputtering to the radial distribution of energy transfers: A molecular dynamics study

Using different models for the deposition of energy on the lattice and a classical molecular dynamics approach to the subsequent transport, we evaluate how the details of the energy deposition model influence sputtering yield from a Lennard-Jones target irradiated with a MeV/u ion beam. Two energy deposition models are considered: a uniform, instantaneous deposition into a cylinder of fixed radius around the projectile ion track, used in earlier molecular dynamics and fluid dynamics simulations of sputtering yields; and an energy deposition distributed in time and space based on the formalism developed in the thermal spike model. The dependence of the sputtering yield on the total energy deposited on the target atoms is very sensitive to the energy deposition model. To clarify the origin of this strong dependence, we explore the role of the radial expansion of the electronic system prior to the transfer of its energy to the lattice. The results imply that observables such as the sputtering yield may be used as signatures of the fast electron-lattice energy transfer in the electronic energy-loss regime, and indicate the need for more experimental and theoretical investigations of these processes.

Chemical Dynamics Simulations of Energy Transfer in Collisions of Protonated Peptide−Ions with a Perfluorinated Alkylthiol Self-Assembled Monolayer Surface

Classical trajectory simulations are performed to study energy transfer in collisions of protonated diglycine, gly2-H+, and dialanine, ala2-H+, ions with a fluorinated octanethiol self-assembled monolayer (F-SAM) surface for collision energies Ei in the range of 5−70 eV and incident angles θi of 0 and 45° with respect to the surface normal. Both explicit-atom (EA) and united-atom (UA) models were used to represent the F-SAM surface. The simulations show the distribution of energy transfer to the peptide−ion’s internal degrees of freedom, ΔEint, to the surface, ΔEsurf, and in peptide−ion translation, Ef, are very similar for gly2-H+, and ala2-H+. The average percentage energy transferred to ΔEsurf and Ef increases and decreases, respectively, with an increase in Ei, while the average percentage energy transfer to ΔEint is nearly independent of Ei. Changing θi from 0 to 45° decreases and increases the percentage of energy transfer to ΔEsurf and Ef, respectively, but has little change in the transfer to ΔEint. Average percentage energy transfer to the surface is found to approximately depend on Ei according to exp(−b/Ei). Comparisons with previous simulations show that peptide-H+ collisions with the EA F-SAM model transfer approximately a factor of 2 more energy to ΔEint than do collisions with the hydrogenated SAM, that is, H-SAM. Replacing the mass of the F atoms by that of a H atom in the simulations, without changing the potential, shows that the different ΔEint energy transfer efficiencies for the F-SAM and H-SAM surfaces is a mass effect. The simulations for ala2-H+ colliding with the EA F-SAM surface give P(ΔEint) distributions in good agreement with previous experiments and an average transfer to ΔEint of 15% as compared with the experimental value of 21%. The UA F-SAM model gives energy transfer efficiencies in qualitative agreement with those of the EA model, but there are important quantitative differences.