Search

Saliva is a body fluid that is much easier to collect and analyze than blood. Thus, saliva analysis for the detection of delta 9‐tetrahydrocannabinol (delta 9‐THC) can serve as a tool for law enforcement agents to detect cannabis consumption by drivers. Fast saliva analysis for the presence of delta 9‐THC and/or cannabidiol (CBD) is described with both gas chromatography–mass spectrometry (GC–MS) with Cold electron ionization (EI) with good separation and in 10 min and/or with Open Probe Fast GC–MS with Cold EI in under 1 min full analysis cycle time. Saliva was taken directly from donors' tongues on a thin glass rod that was used "as is" for analysis. The saliva was thermally desorbed with a modified ChromatoProbe device inside the gas chromatograph (GC) injector and in an Open Probe (Agilent name QuickProbe) for its sub‐1‐min analysis. Cold EI is based on coupling of the GC and mass spectrometer (MS) with a supersonic molecular beam and on EI of vibrationally cold sample molecules during their flight through a contact‐free ion source (thereby named Cold EI). A revised type of Open Probe Fast GC–MS on the bench is also described. Our saliva analysis was characterized by: Saliva can be collected in the field and transported to the lab for analyses "as is" without any sample preparation.Easy detection of cannabis consumption from cigarettes and/or other cannabis products.Distinction between the isomers delta 9‐THC and CBD.Ultra‐fast analysis in under 1 min using Open Probe Fast GC–MS with Cold EI. [ABSTRACT FROM AUTHOR]

Gas chromatography–mass spectrometry (GC–MS) with Cold EI is based on interfacing GC and MS with supersonic molecular beams (SMBs) along with electron ionization of vibrationally cold sample compounds in SMB in a fly‐through ion source (hence the name Cold EI). Cold EI improves all the central performance aspects of GC–MS, and in this paper, we focus on its improvement of signal‐to‐noise ratio (S/N) and limits of detection (LODs). We found that the harder the compound for analysis with standard EI, the greater the Cold EI gain in S/N and LOD. The lower LOD and higher S/N of Cold EI emerge from a few reasons: (a) similar ionization yield as standard EI, (b) enhanced abundance of molecular ions, (c) elimination of vacuum background noise, (d) elimination of ion source‐related peak tailing and degradation, (e) ability to lower the elution temperatures via the use of high column flow rates, and (f) greater range of thermally labile and low‐volatility compounds that can be analyzed. We demonstrate the superior S/N and lower LOD of Cold EI versus standard EI in a range of compounds, from the simple‐to‐analyze octafluoronaphthalene all the way to reserpine and an organo‐metallic compound that cannot be analyzed by standard EI. These compounds include methyl stearate, cholesterol, n‐C32H66, large polycyclic aromatic hydrocarbons, dioctyl phthalates, diundecyl phthalate, pentachlorophenol, benzidine, lambda‐cyhalothrin, and methidathion. The significantly lower Cold EI LODs that can be over 1000 times better than in standard EI further result in far superior response linearity and greater measurement dynamic range. [ABSTRACT FROM AUTHOR]

Open Probe is based on a vaporization oven mounted on a transfer line of a gas chromatograph mass spectrometer (GC/MS) which is connected to the MS ion source via a short flow restriction capillary. The probe oven is open to room air while having helium purge flow protection to eliminate or significantly reduce air leakage into the oven and MS ion source. Sample analysis can be as simple as touch (the sample), push (the sample holder) into the open probe oven, and have the results. Experiments were performed with a GC/MS with supersonic molecular beams and with a standard Agilent 5975 MSD. Powders and tablets analysis were performed by touching the sample with the bottom side of a melting point vial and inserting it into the open probe oven with total analysis cycle time of 10.1021/ac100834h

This study covers a new method and related instrumentation for whole blood analysis for medical diagnostics. Two‐μL whole blood samples were collected using "minimal invasive" diabetes lancet and placed on a thin glass rod mounted on a newly designed BloodProbe. The BloodProbe with the whole blood sample was inserted directly into a ChromatoProbe mounted on the GC inlet, and thus, no sample preparation was involved. The analysis was performed within 10 min using a GC‐MS with Cold EI that is based on interfacing GC and MS with supersonic molecular beams (SMB) along with electron ionization of vibrationally cold sample compounds in the SMB (hence the name Cold EI). Our blood analysis revealed several observations: (1) Detailed mass chromatograms were generated with full range of all the nonpolar lipids in blood including fatty acids, cholesterol, cholesteryl esters, vitamin E, monoglycerides, diglycerides, and triglycerides. (2) The analysis of whole blood was found to be as informative as the conventional clinical analysis of blood serum. (3) Cholesteryl esters were more sensitive than free cholesterol alone to the effect of diet of obese people. (4) Major enhancement of several fatty acid methyl esters was found in the blood of a cancer patient with liver dysfunction. (5) Vitamin E as both α‐ and β‐tocopherol was found with person‐dependent ratio of these two compounds. (6) Elemental sulfur S8 was identified in blood. (7) Several drugs and other compounds were found and need further study of their correlation to medical issues. [ABSTRACT FROM AUTHOR]

Gas chromatography–mass spectrometry (GC‐MS) with Cold EI is based on interfacing GC and MS with a supersonic molecular beam (SMB) and sample compounds ionization with a fly‐through ion source as vibrationally cold compounds in the SMB (hence the name Cold EI). We explored the use of nitrogen and hydrogen as carrier and make‐up gases with Cold EI and found:Nitrogen is very effective in cooling compounds in SMB and while helium requires 60 ml/min nitrogen provides effective cooling with only 7–8 ml/min combined column and make‐up flow rate. Hydrogen is less effective than helium and requires higher flow rates.The transition from helium to nitrogen (or hydrogen) is simple and fast and requires just closing the helium valve and opening the nitrogen valve.The same column used with helium can be used with nitrogen or hydrogen.The same elution times could be obtained with nitrogen or hydrogen as with helium.The GC separation with nitrogen was reduced compared with helium and peak widths were increased by an average factor of 1.5 for similar elution times. Hydrogen provided ~0.7 narrower peak widths than helium.The signal with nitrogen was reduced compared with helium by an average factor of 3.3 and the signal loss was reduced with higher compounds mass. With hydrogen the signal loss was about a factor of 1.5 but the baseline noise was higher thus with similar S/N as with nitrogen.USEPA 8270 semivolatile mixture was easily analyzed with both nitrogen and hydrogen carrier gases. [ABSTRACT FROM AUTHOR]

GC-MS usually employs a 70 eV electron ionization (EI) ion source, which provides mass spectra with detailed fragment ion information that are amenable for library search and identification with names and structures at the isomer level. However, conventional EI often suffers from low intensity or the absence of molecular ions, which reduces detection and identification capabilities in analyses. In an attempt to enhance the molecular ions, several softer ion sources are being used to supplement standard EI, including chemical ionization (CI), atmospheric pressure chemical ionization (APCI), field ionization (FI), photoionization (PI), and low electron energy EI. However, the most advantageous way to enhance molecular ions is to use cold EI, which employs 70 eV EI of cold molecules in supersonic molecular beams. Cold EI yields classical EI mass spectra with highly enhanced molecular ions, which still provides high detectability and library-searchable mass spectra. In this paper, we explain and discuss why cold EI is not a supplementary ion source to standard EI, but rather it is a highly superior replacement to standard EI. With cold EI, there is no need for standard EI or any other supplemental ion source. We describe 16 benefits and unique features of cold EI that not only yield better results for existing applications but also significantly extend the range of compounds and applications amenable for GC-MS analysis. [ABSTRACT FROM AUTHOR]

Cannabis extracts and products were analyzed by gas chromatography–mass spectrometry (GC–MS) with Cold EI for their full content including terpenes, sesquiterpenes, sesquiterpinols, fatty acids, delta 9‐tetrahydrocannabinol (THC), cannabidiol (CBD), other cannabinoids, hydrocarbons, sterols, diglycerides, triglycerides, and impurities. GC–MS with Cold EI is based on interfacing GC and MS with supersonic molecular beams (SMB) along with electron ionization of vibrationally cold sample compounds in the SMB in a fly‐through ion source (hence the name Cold EI). GC–MS with Cold EI improves all the performance aspects of GC–MS, enables the analysis of Cannabinoids with OH groups without derivatization, while providing enhanced molecular ions for improved identification, and enables internal quantitation without calibration. We found over 50 cannabinoid compounds including a new one with a Cold EI mass spectrum very similar to delta 9‐THC as well as relatively large cannabinoids with molecular weight above m/z = 400. Because the analysis was universal in full scan and not targeted, we found impurities such as bromo CBD and fluticasone propionate and could monitor the formation of oxidized CBD during decarboxylation. In addition, GC–MS with Cold EI enabled nontargeted full analysis of terpenes, sesquiterpenes, and sesquiterpinols in cannabis extracts with good internal quantitation. GC–MS with Cold EI further served with very good sensitivity for the concentration determination of delta 9‐THC in CBD‐related products. Finally, cannabis drugs such as EP‐1 used in Israel for treatment of epilepsy and for children with autism spectrum disorder (ASD) were analyzed for their full cannabinoids content for learning on the entourage effect and for drug activity optimization. [ABSTRACT FROM AUTHOR]

Electron ionization (EI) mass spectra of 46 compounds from several different compound classes were measured. Their molecular ion abundances were compared as obtained with 70-eV EI, with low eV EI (such as 14 eV), and with EI mass spectra of vibrationally cold molecules in supersonic molecular beams (Cold EI). We further compared these mass spectra in their National Institute of Standards and Technology (NIST) library identification probabilities. We found that 1. Low eV EI is not a soft ionization method, and it has little or no influence on the molecular ion relative abundances for large molecules and those with weak or no molecular ions. 2. Low eV EI for compounds with abundant or dominant molecular ions in their 70 eV mass spectra results in the reduction of low mass fragment ions abundances thereby reducing their NIST library identification probabilities thus rarely justifies its use in real-world applications. 3. Cold EI significantly enhances the relative abundance of the molecular ions particularly for large compounds; yet, it retains the low mass fragment ions; hence, Cold EI mass spectra can be effectively identified by the NIST library. 4. Different standard EI ion sources provide different 70 eV EI mass spectra. Among the Agilent technologies ion sources, the "Extractor" exhibits relatively abundant molecular ions compared with the "Inert" ion source, while the "High efficiency source" (HES) provides mass spectra with depleted molecular ions compared with the "Inert" ion source or NIST library mass spectra. These conclusions are demonstrated and supported by experimental data in nine figures and two tables. [ABSTRACT FROM AUTHOR]

Pharmaceuticals require careful and precise determination of their impurities that might harm the user upon consumption. Although today, the most common technique for impurities identification is liquid chromatography-mass spectrometry (LC-MS/MS), it has several downsides due to the nature of the ionization method. Also, the analyses in many cases are targeted thus despite being present, some of the compounds will not be revealed. In this paper, we propose and show a new method for untargeted analysis and identification of impurities in active pharmaceutical ingredients (APIs). The instrument used for these analyses is a novel electron ionization (EI) LC-MS with supersonic molecular beams (SMB). The EI-LC-MS-SMB was implemented for analyses of several drug samples spiked with an impurity. The instrument provides EI mass spectra with enhanced molecular ions, named Cold EI, which increases the identification probabilities when the compound is identified with the aid of an EI library like National Institute of Standards and Technology (NIST). We analyzed ibuprofen and its impurities, and both the API and the expected impurity were identified with names and structures by the NIST library. Moreover, other unexpected impurities were found and identified proving the ability of the EI-LC-MS-SMB system for truly untargeted analysis. The results show a broad dynamic range of four orders of magnitude at the same run with a signal-to-noise ratio of over 10 000 for the API and almost uniform response. [ABSTRACT FROM AUTHOR]

A new type of photoionization ion source was developed for the ionization of cold molecules in supersonic molecular beams (named Cold PI). The system was based on a GC–MS with supersonic molecular beams and its fly-through EI of cold molecules ion source (Cold EI) plus quadrupole mass analyzer. A continuously operated deuterium VUV photoionization lamp was added and placed above and between the supersonic nozzle and skimmer whereas the Cold EI ion source served only as a portion of the ion transfer ion optics. The supersonic nozzle and skimmer were voltage biased and the VUV light crossed the supersonic expansion about 10 mm from the nozzle. We obtained over three orders of magnitude enhancement in the relative abundance of the molecular ion of squalane in Cold PI versus in photoionization of this compound as a thermal compound. Accordingly, we also proved that standard photoionization is not as soft ionization method as previously perceived for large compounds. We found that Cold PI is as soft as and possibly softer than field ionization; thus, it could be the softest known ionization method. The ionization yield was about 200–300 times weaker than with Cold EI yet our limit of detection was about 200 femtogram in SIM mode for cholesterol and pyrene which is reasonable. Practically, all hydrocarbons gave only molecular ions with rather uniform response whereas alcohols gave some molecular ions plus major fragment ions particularly with a loss of water (similarly to field ionization). We tested Cold PI in the GC–MS analysis of diesel fuels and analyzed the time averaged data for group type information. We also found that we can analyze the diesel fuels by fast under 20-s flow injection analysis in which the generated averaged mass spectrum of molecular ions only could serve for the characterization of fuels. [ABSTRACT FROM AUTHOR]

A spray based-method for the formation and production of semiconductor nanocrystals that provides an attractive alternative to the commonly used epitaxial and colloidal procedures is presented. A semiconductor nanocrystal is produced from a single spray droplet upon the full vaporization of the solvent.

The heavy petroleum fractions produced during refining processes need to be upgraded to useable products to increase their value. Hydrogenated heavy paraffinic fractions can be oxidised to produce high value products that contain a variety of oxygenates. These heavy oxygenated paraffinic fractions need to be characterised to enable the control of oxidation processes and to understand product properties. The accurate identification of the oxygenates present in these fractions by electron ionisation (EI) mass spectrometry is challenging due to the complexity of these heavy fractions. Adding to this challenge is the limited applicability of EI mass spectral libraries due to the absence of molecular ions from the EI mass spectra of many oxygenates. The separation of oxygenates from the complex hydrocarbon matrix prior to high temperature GC-MS (HT-GC-MS) analysis reduces the complexity of these fractions and assists in the accurate identification of these oxygenates. Solid phase extraction (SPE) and supercritical fluid chromatography (SFC) were employed as prefractionation techniques. GC-MS with supersonic molecular beams (SMBs) (also named GC-MS with cold-EI) utilises a SMB interface with which EI is done with vibrationally cold sample compounds in a fly-through ion source (cold-EI) resulting in a substantial increase in the molecular ion signal intensity in the mass spectrum. This greatly enhances the accurate identification of the oxygenates in these fractions. This study investigated the ionisation behaviour of oxygenated compounds using cold-EI. The prefractionation by SPE and SFC and the subsequent analysis with GCMS with cold-EI were applied to an oxygenated heavy paraffinic fraction. [ABSTRACT FROM AUTHOR]

Clusters of the form DCA(R1R2), where DCA=dichloroanthracene and R1, R2=Ar, Kr, and Xe were synthesized in a supersonic molecular beam. The mixed clusters were efficiently formed by the coexpansion of DCA in a mixture of two different rare gases with the heavier rare gas being in the minority. The clusters’ vibrational predissociation dynamics was probed using a nanosecond excimer pumped dye laser spectroscopy and energy resolved emission diagnostics. DCA was chosen for this study because of its high emission quantum yield and relatively few spectral interferences at high vibrational energy. The emission quantum yield of DCA–rare gas atom complexes was found to be unity at the electronic origin. The emission quantum yield is greatly reduced upon the increase of the vibrational energy being 0.052 at the 1390 cm-1 vibrational level. It was more than three times higher in the DCA–rare gas atom clusters at the 1390 cm-1 vibration, in comparison with that of the bare molecule, due to vibrational predissociation. The vibrational predissociation products have been identified using energy resolved emission. At 1390 cm-1, excess vibrational energy two argon atoms or one xenon atom could dissociate. For DCA(Kr)n, it is not clear whether one or two krypton atoms had dissociated. In DCA(XeAr) or DCA(XeKr) excited to the 1390 cm-1 vibration, either one of the two rare gas atoms could dissociate, but not both of them.The results indicate that predominantly the weakest bound rare gas atom dissociates, although its vibrational modes seems less effectively coupled to the excited skeleton modes. The dissociation rates were determined by the relative emission intensity before and after the dissociation, which could be spectrally identified. The time scales for vibrational predissociation of all the various DCA clusters were found to be about 1 ns, independent of the rare gas identity. The results are interpreted by assuming the excitation of a vibrationally... [ABSTRACT FROM AUTHOR]

The absorption spectra of jet-cooled bromoiodomethane (CH2BrI), 1,2-bromoiodoethane (Br(CH2)2I) and 1,3-bromoiodopropane (Br(CH2)3I) were measured in the spectral range 1600–2100 Å, and were compared with the corresponding monohalide absorption spectra. A pronounced broadening of the halogen nonbonding→Rydberg transition was observed in CH2BrI and in Br(CH2)2I whereas the transition width was considerably smaller in Br(CH2)3I. These results are rationalized in terms of a mixing of the halogen Rydberg states and the C–X (X=Br,I) antibonding state. The photochemical implications of these spectra are discussed in connection with a possible Rydberg state selective photochemistry. [ABSTRACT FROM AUTHOR]

The solvation effects on the molecular pure radiative lifetime, its absorption line shape, oscillator strength, and spectral red shift are studied for 9, 10-dichloranthracene embedded in clusters of argon, krypton, and xenon. The clusters are synthesized in a supersonic free-jet coexpansion of the organic molecule with the rare gas. Cluster size is controlled by the nozzle backing pressure and its pure radiative lifetime is obtained by measuring both the fluorescence lifetime and the absolute emission quantum yield. For small (jet atom) clusters the pure radiative lifetime is increased by subsequently adding rare gas atoms. For up to two rare gas atoms this increase correlates with the atomic polarizabilities and with the spectral red shift. For Ar and Kr this trend of increasing radiative lifetime continues up to clusters of six rare gas atoms. These results are in good agreement with calculations based on classical electromagnetic theory using point polarizable dipoles for the atom and a simplified charge distribution for the molecule. For large clusters of Ar (up to ∼1000 atoms) both the spectral red shift and the lifetime become cluster size independent. This is also in agreement with classical electromagnetic theory using a model of a point (molecular) dipole embedded in a dielectric (rare gas) sphere. Both experiment and theory indicate that the radiative lifetime in this limit is still larger than that of the free molecule, which in itself is longer than that expected in the bulk solvent. This implies that a further cluster size evolution of this quantity is expected upon increasing the cluster size above the radiation wavelength. We also report on a sudden lineshape broadening and a slight spectral blue shift that accompanies the growth of large molecule–argon clusters and which we interpret as originating from a possible transition towards the surface of the molecule in order to minimize growth strain. [ABSTRACT FROM AUTHOR]

We have measured the absolute rates of internal conversion and of intersystem crossing in isoquinoline excited to its S2 vibronic states (3131–3062 Å) in supersonic beams. The combination of direct detection of the triplets, absorption and excitation spectra allowed us to extract these rates. The intersystem crossing rate barely depends on the excess vibrational energy in the S2 manifold, being about 2×109 s-1. The internal conversion rates grow by more than two orders of magnitude over the range of 0–800 cm-1 of excess vibrational energy in the S2 state, up to 5×1010 s-1. At the S2 electronic origin intersystem crossing is the dominant nonradiative channel, while at vibrational energy exceeding 400 cm-1 internal conversion becomes dominant. [ABSTRACT FROM AUTHOR]

The absorption and fluorescence excitation spectra of jet cooled Cd(CH3)2 and Cd(CD3)2 were measured in the structured region between 230 and 200 nm. The symmetric Cd–methyl stretch and methyl umbrella modes in the second excited electronic state of the latter molecule were found to the 304 and 790 cm-1, respectively. Widths of the 0–0 lines (FWHM) were 101 and 25 cm-1, respectively from which lifetimes of 53 and 265 fs were deduced. A weak fluorescence due to CdCH3 is excited by the absorption of Cd(CH3)2 and an even weaker fluorescence is excited in Cd(CD3)2. The quantum yield of this fluorescence is below 10-4 at the electronic origin of the structured band and increases to about 10-3 above 2000 cm-1 vibrational energy. [ABSTRACT FROM AUTHOR]

In this paper we explore the energy dependence of the interstate electronic relaxation rates knr from the S1 manifold of anthracene and seven of its isotopic and chemical derivatives, which were inferred from quantum yield data. Absolute fluorescence quantum yields Y from groups of rotational states within the electronic origin S1(0) and from vibrational states were obtained over the excess energy Ev=0–3000 cm-1 above S1(0) by the simultaneous interrogation of the fluorescence excitation spectra and of the absorption spectra in seeded, pulsed, planar supersonic jets of Ar. Additional information was obtained from quantum yield data of van der Waals (vdW) complexes of these molecules with Ar. The fluorescence quantum yields from the S1(0) of anthracene, 9-cyano-anthracene, and 9,10-dibromoanthracene were found to be independent of the rotational state, providing further evidence for the rotational independence of knr from a single doorway state. From the Y data of the electronic origins and from the Ev dependence of Y we conclude that intersystem crossing (ISC) dynamics of the S1 manifold is dominated by the interplay between two classes of nonreactive coupling and/or relaxation: (i) Interstate coupling, involving the superposition of direct S1→(T1) ISC together with S1→(Tx) →(T1) mediated ISC through a sparse or dense (Tx) manifold of a higher triplet state; (ii) Intrastate coupling within the S1 manifold, which sets in with increasing Ev and which results in intramolecular vibrational energy redistribution (IVR) at high Ev. The dominant role of mediated interstate coupling in ISC dynamics from S1(0) and from low Ev states was inferred from the inverse deuterium isotope effect on the ISC rates, the extreme sensitivity of knr of deuterated anthracene to a single H atom substitution, and to level shifts induced by complexing with Ar, as well as from the three-orders-of-magnitude difference between the knr values from the S1(0) of... [ABSTRACT FROM AUTHOR]

In this paper we report on the absolute fluorescence quantum yields, Y, from photoselected rotational and vibrational states within the S1 manifold of pyrazine cooled in planar, pulsed, supersonic expansions of Ar. The Y data from the S1 origin exhibit a marked J’ dependence, which can be fit by the empirical relation Y=0.124/(2J’+1) for J’=5–22. The vibrational state dependence of Y on the excess vibrational energy, EV, exhibits an approximate exponential decrease with increasing EV in the range EV=0–1300 cm-1 with a possible saturation at EV=1300–1800 cm-1. The low absolute Y values can be attributed to intramolecular nonradiative decay of S1–T molecular eigenstates. The marked rotational state dependence of Y from the S1 origin can be interpreted in terms of the intermediate level structure (ILS) provided that the dilution factor is Neff=16(2J’+1) for J’=5–22. This Neff exhibits a qualitatively different J’ dependence than the number of effectively coupled states originating from a single J’K’ doorway state N(spectroscopic)=12–15 for J’=0–3, which was inferred by Kommandeur et al. [Chem. Phys. Lett. 92, 565 (1980)] from high resolution spectroscopic studies. In order to reconcile our Y data, together with the available psec time-resolved data and the spectroscopic information in terms of the ILS model, one has to assert that the range J’=3–5 marks a crossover from J’ independent to J’ dependent Neff. [ABSTRACT FROM AUTHOR]

In this paper, we present an electrostatic theory of the effect of small atomic cluster environment on molecular oscillator strengths and spectra. The molecular dipole is represented by a classical oscillating charge distribution and the cluster atoms by spherical polarizable particles. From our general theoretical results, we calculate the effect of cluster size and geometry on the molecular radiative lifetime and the spectral shift. These properties exhibit a considerable sensitivity to the cluster and molecular geometry, i.e., the cluster size and structure, the molecule–atom(s) distance(s), the spatial size of the molecular transition dipole, and molecular orientation within the cluster. The reduced plot of the relative change in the oscillator strength vs the relative frequency shift seems to have universal character and is useful in analyzing and predicting experimental trends. Available experimental results are consistent with the theoretical predictions. [ABSTRACT FROM AUTHOR]

We present the results of stochastic classical trajectory simulations of the scattering of a nonreactive gas, xenon, from a semiconductor surface of known structure and electronic and vibrational properties, GaAs(110). The range of incident energies considered is 1 to 8 eV, in order to make direct comparison with results of molecular beam experiments reported in the accompanying paper. We employed a 48-atom three-layer slab of GaAs, with periodic boundary conditions in two dimensions and frictional and stochastic forces in the third (surface normal) dimension. Pairwise additive Lennard-Jones potentials describe the gas–surface interaction. The calculations reproduce the large energy exchange and surprising structural sensitivity observed experimentally. Energy transfer is dominated by an initial binary interaction of the Xe with a single Ga or As atom. The repetitive collision nature of this binary encounter produces angular scattering patterns very similar to those obtained for scattering of light atoms, including ‘‘rainbow maxima.’’ The mechanism of energy deposition may have implications to ‘‘gentle’’ sputtering by neutral atom bombardment and to collision induced excitation of electron–hole pairs. [ABSTRACT FROM AUTHOR]