R. J. Beuhler

A study of the formation of high molecular weight water cluster ions (m/e<59 000) in expansion of ionized gas mixtures

Mass spectrometric determinations have been made of size distributions of cluster ions produced by expansion of ionized mixtures of water and various carrier gases. Measurements span a range up to mass to charge ratios of 59 000. Dilute water–nitrogen carrier gas mixtures producing cluster masses up to 12 000 gave size distributions which are first order in water concentration. This observation is explained by the relatively large value of the ion–dipole associative collision cross sections which determined the rate of growth of cluster ions in the mass range studied. Studies in different mass carrier gases show that cluster ion size distributions are largely determined by the cross section for, and number of, associative collisions taking place during the expansion process. Estimates of associative collision efficiency have been made for water and alcohol cluster ion growth on expansion. Evidence has been obtained for steric inhibition of efficient alcohol collision processes compared to relatively effic...

A model of secondary electron yields from atomic and polyatomic ion impacts on copper and tungsten surfaces based upon stopping‐power calculations

The calculation of the velocity dependence and magnitude of the kinetic secondary electron yield for atomic and polyatomic ions, denoted by γ a and γ m , respectively, is modeled on stopping‐power equations and compared to experimental results from the dynode materials copper and tungsten. From the experimental data, the relationship γ m =Σγ a is verified, which is necessary to the model calculation of γ m . The fundamental assumption of this model is that γ a is proportional (even at low ion velocities) to the electronic stopping power (d E/d x) e of that projectile in the target. This fundamental assumption of γ a ∝ (d E/d x) e has been previously used by Sternglass for calculation of γ a ’s at ion velocities above 4×108 cm/sec. Example calculations are performed for the velocity dependence of γ a for hydrogen, carbon, and fluorine striking copper, and from these results the predicted γ m curves for polyatomic ions are in close agreement with experiment. Analogous calculations for a tungsten surface are outlined and the final results are compared with experiment. The model’s utility is shown by illustrating that the measurement of γ can provide not only a relative measurement of (d E/d x) e for different ion atomic numbers on the same target, but can also provide a means of determining the nuclear stopping power (d E/d x) n at low ion velocities where range measurements are difficult to perform. Values of (d E/d x) n derived from measurements for fluorine and carbon striking copper are the same as values predicted by Lindhard and Scharff; for a tungsten target the experimental (d E/d x) n values are a factor of 2 larger than theoretical. The relationship of the stopping power values derived from γ measurements with this model are compared to the values derived from ion range measurements. In addition, the model’s relationship to the secondary electron emission model of Parilis and Kishinevskii is discussed.

Hydrogen cluster ion equilibria

The equilibria H3++H2=H5+ and H5++H2=H7+ have been studied using both hydrogen and deuterium in a cooled corona discharge ion source fitted with a very small ion exit aperture. Enthalpies of reaction are in relatively good agreement with results obtained in earlier studies by Hiraoka and Kebarle using a pulse electron impact ion source. Absolute values of equilibrium constants and entropies of reaction are in somewhat poorer agreement. The enthalpy determined for the reaction yielding H5+ was found to be in slightly better agreement with the recent theoretical study of Yamaguchi, Gaw, and Schaeffer than results obtained in earlier examinations. A discussion of potential errors in the experimental determination of these equilibria is presented.

A comparison of secondary electron yields from accelerated water cluster ions (M/z<50 000) striking Al2O3 and copper surfaces

A comparison of the yields of secondary electrons (γ) ejected by water cluster ions, with M/z ratios from 18 to 49 000 and kinetic energies from 40 to 250 kV, striking copper and aluminum oxide surfaces is detailed. For clusters striking Al2O3 with M/z in the range 21 000–49 000, threshold velocities of 1×106 cm/sec were observed, lower than previously measured values from Cu. Threshold for secondary electron emission for cluster ions occurs when the estimated depth of penetration, based on standard stopping power formulae, is less than one atomic layer. For the oxide surface it is shown that the electron yields do not obey an additive relationship. The yields are always less than one would predict by summing the electron yields from each water molecule in the cluster. Lack of additivity is also shown in the yields resulting from small water clusters striking Cu. Electron yields from both surfaces are compared to a model which relates γ to the energy deposited in the target within the electron escape dept...

Magic numbers for argon and nitrogen cluster ions

Evidence for the existence of magic numbers for cluster ions of argon atoms and nitrogen molecules has been found with high molecular weight mass spectra of their cluster ions. The observed numbers of 145, 303, 550, and 912 in mass spectra of cluster ions coincides within experimental error limit with the number of particles required for complete shell icosahedra. Failure to observe similar magic number structures in mass spectra of polyatomic molecule cluster ions and differences between polyatomic molecule cluster ion mass spectra and the argon and nitrogen cluster ion spectra are discussed.

Volatility enhancement of thyrotropin releasing hormone for mass spectrometric studies

Summary Effects of ionization chamber surfaces on volatility of Thyrotropin Releasing Hormone (TRH) have been investigated in a tandem mass spectrometer. Low velocity CH 3 NH 3 + ions were used as ionizing reagent in single collision processes. Temperature dependences of quasi-parent (TRH + Proton) ion yields were used to determine activation energies of sample volatilization. Activation energies of desorption from Teflon surfaces were 30 kcal/mole lower than values obtained with powder samples and samples dispersed on glass from dilute solutions. Ion impact spectra obtained with 2 μg samples of TRH on Teflon gave approximately 50% of total ions as quasi-parent and sufficient fragmentation to establish the amino acid sequence. Spectra are shown to be both temperature and surface sensitive. Volatility enhancement by dispersion of samples on selected surfaces facilitates study of small peptides with minimal chemical derivatization and should extend the range of sizes of derivatized molecules which may be evaporated in mass spectrometer ion sources.

State-resolved photodesorption of CO from Ag(111): Nonthermal desorption at 1064 nm

State-resolved measurements of CO molecules desorbed from Ag(111) by 1064 nm radiation have been performed using (VUV+UV) resonant multiphoton ionization. The CO translational and rotational energies are found to be essentially constant over a wide range of 1064 nm laser fluences indicating a non-thermal desorption mechanism.

Kinetics of volatilization of hydrogen bonding species: Comparison of experimental and calculated frequency factors for N-acetyl-(ℓ-alanine) methyl esters

Abstract Observed relative frequency factors for the volatilization of N-acetyl-(l-alanine) methyl esters are compared with those calculated on the basis of a high-pressure unimolecular rate constant derived by Schlag for multiple critical oscillators. The comparison suggests that this model for unimolecular reactions gives a good qualitative picture of the kinetics of volatilization of thermally labile, hydrogen-bonded species.

Nonthermal photodesorption of N2 from Ag(111)

We have measured translational and rotational energy distributions of N2 molecules following desorption from a Ag(111) surface by infrared (1064 nm) radiation. The observed desorption yields were large even at laser fluences far below that required for laser-induced thermal desorption. State-resolved laser techniques using coherent VUV radiation showed that the rotational and translational energy distributions of the desorbing N2 molecules are not consistent with the predictions of the heat diffusion model governing laser-induced surface heating. These results suggest that physisorbed adsorbates can couple directly to the nascent-phonon distribution or the nascent electron-hole pairs in the photoexcited substrate without heating of the surface.