Cyclotron Resonance Study Research Articles

A fourier transform ion cyclotron resonance study of two- and one-electron capture reactions between doubly charged rare gas ions and rare gas atoms

A Fourier transform ion cyclotron resonance (FTICR) mass spectrometer has been used to study near thermal collision energy symmetric charge transfer reactions. The systems Xe n+ /Xe, Kr n+ /Kr and Ar n+ /Ar have been measured at ambient temperature, where n = 1, 2. In the interactions between doubly charged rare gas ions and their corresponding neutral gas atoms the double electron capture (DEC) has been observed to be more efficient than the single electron capture (SEC). From the measured overall rate coefficient of 3.4 × 10 −10 cm 3 s −1 and the branching ratio of 1.1 between the DEC reaction and the SEC reaction the rate coefficients for double and single electron transfer from Xe to Xe 2+ have been calculated to be 1.8 × 10 −10 cm 3 s −1 and of 1.6 × 10 −10 cm 3 s −1. In the experiments with doubly charged krypton and doubly charged argon ions in their parent gases the symmetric double charge transfer is observed to be even more favored in comparison with the Xe 2+/Xe system at near thermal collision energies.

Fourier transform ion cyclotron resonance study of multiply charged aggregates of small singly charged peptides formed by electrospray ionization

Aggregates of singly protonated peptides formed with a nanoelectrospray ion source have been observed in the gas phase using Fourier transform ion cyclotron resonance (FT-ICR). Employment of “soft” ion sampling conditions in the source, which were developed previously to generate water clusters of biomolecules, provides significant yields of aggregates of singly protonated GGDPG ([2GGDPG + 2H] 2+), GGEPG ([2GGEPG + 2H] 2+), and VEPIPY (2VEPIPY + 2H] 2+). With peptide mixtures, heteroaggregates, e.g., [GGDPG + GGEPG + 2H] 2+ have also been observed along with the homoaggregates. These weakly bound noncovalent complexes undergo facile exothermic dissociation into the corresponding singly protonated monomer species with normal operation of the electrospray ion source. For example, the aggregates were not observed in FT-ICR experiments utilizing a conventional electrospray ionization (ESI) or fast atom bombardment source or with a quadrupolar ion trap mass spectrometer equipped with a conventional ESI source. The formation and metastability of these aggregates are dependent on highly specific intermolecular hydrogen bonding between the monomers. The amino acid sequence (DPG) of GGDPG mimics the well-known β reverse turn of proteins and semiempirical calculations show that it provides excellent hydrogen bonding sites for a protonated N-terminus amino group. Support for this conjecture is provided by the failure to observe aggregate formation of singly protonated peptides with several larger peptides, including hexaglycine and hexaalanine.

High-field cyclotron resonance and electron–phonon interaction in modulation-doped multiple quantum well structures

A systematic study of electron cyclotron resonance (CR) in two sets of GaAs/Al0.3Ga0.7As modulation-doped quantum-well samples (well widths between 12 and 24 nm) has been carried out in magnetic fields up to 30 T. Polaron CR is the dominant transition in the region of GaAs optical phonons for the set of lightly doped samples, and the results are in good agreement with calculations that include the interaction with interface optical phonons. The results from the heavily doped set are markedly different. At low magnetic fields (below the GaAs reststrahlen region), all three samples exhibit almost identical CR which shows little effect of the polaron interaction due to screening and Pauli-principle effects. Above the GaAs LO-phonon region (B>∼23 T), the three samples behave very differently. For the most lightly doped sample (3×1011cm−2) only one transition minimum is observed, which can be explained as screened polaron CR. A sample of intermediate density (6×1011cm−2) shows two lines above 23 T; the higher frequency branch is indistinguishable from the positions of the single line of the low density sample. For the most heavily, doped sample (1.2×1012cm−2) there is no evidence of high frequency resonance, and the strong, single line observed is indistinguishable from the lower branch observed from sample with intermediate doping density. We suggest that the low frequency branch in our experiment is a magnetoplasmon resonance red-shifted by disorder, and the upper branch is single-particle-like screened polaron CR.

Pressure dependence of the cyclotron resonance modes in high mobility two-dimensional hole systems in GaAs–(Ga,Al)As heterojunctions

We have performed the first cyclotron resonance studies of high mobility two-dimensional hole systems (2DHS) under pressure. Experiments were carried out using millimetre wave radiation on GaAs–(Ga,Al)As heterojunctions. Pressures were applied using a clamp cell specially designed for millimetre wave measurements. Our data at ambient pressure show collectively coupled resonances arising from interactions between the two hole systems with different effective masses formed by the spin-split halves of the lowest heavy-hole subband. The application of pressure appears to completely suppress the collective effect and only a resonance corresponding to the lighter cyclotron mass from the heavy-hole spin-split subband is observed.

Resonant magnetopolaron effects in GaAs/AlGaAs multiple quantum well structures

A detailed experimental study of electron cyclotron resonance (CR) has been carried out at 4.2 K in three modulation-doped GaAs/Al 0.3Ga 0.7As multiple quantum well samples in fields up to 30 T. A strong avoided-level-crossing splitting of the CR energies due to resonant magnetopolaron effects is observed for all samples near the GaAs reststrahlen region. Resonant splittings in the region of AlAs-like interface phonon modes of the barriers are observed in two samples with narrower well width and smaller doping concentration. The interaction between electrons and the AlAs interface optical phonon modes has been calculated for our specific sample structures in the framework of the memory-function formalism. The calculated results are in good agreement with the experimental results, which confirms our assignment of the observed splitting near the AlAs-like phonon region is due to the resonant magnetopolaron interaction of electrons in the wells with AlAs-like interface phonons.

Far-infrared cyclotron resonance study of the effect of strain and localisation in Si/SiGe two dimensional electron gases

Far infrared cyclotron resonance measurements have been used to investigate the effective mass ( m*) in the strained silicon channel of modulation-doped, two dimensional electron gases grown on relaxed Si 1− x Ge x . Samples with germanium fractions from 24% to 31% were measured to investigate the influence of strain on m*. Little variation as a function of strain was observed, but for one sample, the resonance position was shifted up in frequency due to localisation effects. This persisted up to higher Landau level filling factors than has been observed previously in other materials systems and was accompanied by a large enhancement in the quantum lifetime.

Cyclotron resonance studies of strongly coupled double quantum wells in tilted magnetic fields near the quantum and semi-classical limits

Far-infrared cyclotron resonance (CR) spectroscopy has been used to study a pair of strongly coupled two-dimensional electron gases (2DEGs) formed in two GaAs quantum wells separated by a thin AlGaAs barrier. The degree of wave function hybridisation, along with the effect of a magnetic field parallel to the plane of the electron gas, have been investigated near both the quantum and semi-classical limits, corresponding to low and high filling factors, respectively. Near the quantum regime, the CR transitions in a small parallel field reveal anticrossing between the Landau levels associated with different hybridised subbands. The energies and intensities of these transitions change with front gate bias, yielding information on the bias dependence of the wave function hybridisation and the subband energy splitting. Near the semi-classical limit and with strong parallel magnetic fields, two CR peaks are observed. The corresponding cyclotron masses are compared to those expected for non-circular Fermi contours created by anticrossing of the parabolic dispersion curves associated with the coupled 2DEGs. Experimental results in both limits are compared with predictions from self-consistent solutions of Poisson's and Schrödinger's equations.

High magnetic field studies of quantum transport and cyclotron resonance on 2D gas in GaN/GaAlN heterojunction

Magnetotransport and cyclotron resonance measurements were performed on Si-doped GaN/GaAlN heterostructure under pulsed magnetic field up to 35 T. The existence of 2D gas in the structure has been confirmed by cyclotron absorption and Shubnikov–de-Haas oscillations measurements under tilted magnetic field. In addition, a strong positive and temperature-dependent magnetoresistance is observed. The transport measurements are analyzed by a model based on two parallel conductive channels.

Structure and Energetics of Protonated ω-Methoxy Alcohols

The proton affinity (PA) of a molecule in the gas phase is an expression of its fundamental basicity and is a possible factor controlling the course of many ion−molecule reactions. The formation of an intramolecular hydrogen bond increases the PA value over that of similarly sized monofunctional molecules and the values of ΔS° are an indication of an intramolecular cyclization which occurs via hydrogen bonding in protonated bifunctional molecules. The first step in exploring these thermochemical properties has been the examination of the experimental proton-transfer equilibria using both ion cyclotron resonance (ICR) and high-pressure mass spectrometric (HPMS) studies. Parallel ab initio molecular orbital (MO) calculations on the protonated species show that the cyclized structures are the most stable species, in agreement with the experimental PA and entropy observations.

Microwave study of surface electrons on helium films in a magnetic field

We report preliminary results of a cyclotron resonance study of surface electrons (SE) on saturated helium films covering a PMMA substrate at T > 1 K. The real and imaginary parts of the dielectric response ∈(k,ω) of the SE are measured at fixed k and ω in B-fields up to 10 T in a 12 GHz cavity. The cyclotron resonance of the SE is determined at different helium film thicknesses dHeand at various electron densities. At small dHewe find significant anomalies in the cyclotron resonance lineshape and position. As dHeincreases the lineshape becomes progressively more symmetric and its peak moves towards the cyclotron field value expected for a free electron. To fit these data we have modified the classical Drude expression, introducing two different relaxation times for the low and high B-field regions. The phenomenological formulas fit the data quite well. A systematic theoretical analysis of these results is in progress.

Cyclotron-resonance studies of strongly coupled double quantum wells in tilted magnetic fields near the quantum and semiclassical limits

Far-infrared cyclotron-resonance (CR) spectroscopy has been used to study a pair of strongly coupled two-dimensional electron gases (2DEG's) which were formed in two GaAs quantum wells and separated by a thin ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ barrier. The degree of wave-function hybridization, along with the effect of a magnetic field parallel to the plane of the electron gas, have been investigated near both the quantum and semiclassical limits, corresponding to low and high filling factors, respectively. Near the quantum regime, the CR transitions in the presence of a small parallel field reveal anticrossing between the Landau levels associated with different hybridized subbands. The energies and intensities of these transitions change with front gate bias, yielding information on the bias dependence of the wave-function hybridization and the subband energy splitting. Close to the semiclassical limit and with strong parallel magnetic fields, two CR peaks are observed. The corresponding cyclotron masses are compared to those expected for noncircular Fermi contours created by anticrossing of the parabolic dispersion curves associated with the coupled 2DEG's. Experimental results in both limits are discussed in the light of predictions from self-consistent solutions of Poisson's and Schroedinger's equations.

Comparative cyclotron resonance and luminescence study of photoexcited particles in mixed type-I–type-II GaAs/AlAs multiple quantum wells

We report on the photoluminescence (PL) and photoinduced microwave absorption (PMA) of the elementary excitations in GaAs/AlAs undoped quantum wells (QW's) in the temperature range $2<{T}_{L}<30\mathrm{K}.$ Mixed type-I--type-II QW's are used, since separate two-dimensional electron and hole gases are formed (in the wide and narrow QW's, respectively) by photoexcitation with densities in the range ${10}^{9}<{n}_{e}<~5\ifmmode\times\else\texttimes\fi{}{10}^{10}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}.$ The PL spectra show transitions due to $(e1:\mathrm{hh}1)1S$ excitons $(X)$ and $[(e1:\mathrm{hh}1)1S+e]$ negatively charged excitons ${(X}^{\ensuremath{-}}).$ The PMA dependence on magnetic field that is applied perpendicularly to the QW planes shows cyclotron resonances (CR) due to electrons and ${X}^{\ensuremath{-}}$ in the wide wells and heavy holes in the narrow wells. The CR traces are analyzed and yield the cyclotron effective mass, momentum relaxation rate, and relative density of the charged particles. The PL and PMA spectra are found to correlate well when observed at the same temperature and microwave power. The electron and ${X}^{\ensuremath{-}}$ masses and relative intensities are analyzed in terms of weak in-plane particle localization. It is found that, at low temperatures, a large fraction of the electrons and ${X}^{\ensuremath{-}}$ are localized by interface potential fluctuations.

Cyclotron resonance and quantum Hall effect studies of the two-dimensional electron gas confined at the GaN/AlGaN interface

We report on high magnetic fields (up to 40 T) cyclotron resonance, quantum Hall effect and Shubnikov-de-Hass measurements in high frequency transistors based on Si-doped GaN–AlGaN heterojunctions. A simple way of precise modelling of the cyclotron absorption in these heterojunctions is presented. We clearly establish two-dimensional electrons to be the dominant conducting carriers and determine precisely their in-plane effective mass to be 0.230±0.005 of the free electron effective mass. The increase of the effective mass with an increase of two-dimensional carrier density is observed and explained by the nonparabolicity effect.

A tandem mass spectrometry and Fourier transform-ion cyclotron resonance study of ionized ethylated acetone and deuterated analogs

The unimolecular dissociation of (CH3)2C+OC2H5 ions (I) and their deuterated analogs, generated by ion-molecule reactions (IMR) in acetone-ethyl iodide mixtures was studied by tandem mass Spectrometry methods. Two significant processes that yielded I ions were identified. The Fourier transform ion cyclotron resonance study showed that the reaction between ionized ethyl iodide and neutral acetone was the principal source of I. This process involved the formation of the stable mixed ionized dimer, [C2H5I·O=C(CH3)2]+· (II), which dissociated by the loss of an I atom. Other important fragmentation pathways of II were the formation of C2H5I+·, (CH3)2CO+·; and (CH3)2COI+ and the loss of CH3CHI·. The major dissociation of I was the loss of C2H4. The activation energy for this reaction was determined by metastable ion appearance energy measurements to be ∼55 kJ mol−1 above the thermochemical minimum. The analysis of the metastable and collision-induced dissociation of D-labeled I showed an unusual time-energy effect on the degree of H/D mixing, with the highest selectivity for the ethene loss [β-H(D)-atom shift] being observed for ions with the lowest internal energies. Collisional excitation could not produce significant H/D mixing among dissociating ions. The results were rationalized by the existence of two species— the classical (2-ethoxypropyl) and nonclassical (proton-bound acetone-ethene pair) isomers of I. The classical structure was originally formed by IMR or from II. The energy barrier for the classical to nonclassical isomerization lay well above the thermochemical threshold for C2H4 loss, providing only limited H-atom mixing in nonclassical ions that were always formed in their dissociative state. The effect of the proton affinity of the carbonyl compound on the H/D mixing in RR′C+OC2H5 ions was studied. It was shown that the selectivity for the ethene loss (β-H-atom shift) generally increased with the increase of the proton affinity of RR′CO. Neutralization-reionization mass spectrometry was applied to a study of (CH3)2C+OR ions, where R = H, I, C2H5. The observation of a recovery signal for the ion I was attributed to the formation of the 2-ethoxypropyl radical. Neutral counterparts of (CH3)2COI+ ions were also generated, being the first example of IO-substituted alkyl radicals.

Optical and electrical properties of 2-dimensional electron gas in GaN/AlGaN heterostructures

Cyclotron resonance studies of two-dimensional electrons confined at the GaN/AlGaN interface are presented. The value of the 2D-electron cyclotron mass is determined and discussed in view of non-parabolicity effects. The influence of non-parabolicity is enhanced by the spatial confinement of electrons and is calculated in the triangular well approximation. After subtraction of non-parabolicity corrections, the same mass 0.226 m 0 with 2% precision is obtained for 2D and bulk electrons in GaN.

Lithium-cation and proton affinities of sulfoxides and sulfones: A fourier transform ion cyclotron resonance study

The kinetic method was used for the quantitative determination of lithium-cation affinities by Fourier transform ion cyclotron resonance. This method was applied to a series of XYSO and XYSO2 compounds. Proton basicities of the SO and SO2 compounds were also determined. When comparison is made between Li+ basicities and proton basicities, a linear regression encompassing XYSO and XYSO2 families suggests that Li+ may be bonded in a similar way to the SO and SO2 moieties, that is, to only one oxygen on the latter. PM3 calculations support this hypothesis.

Cyclotron resonance studies of the molecular conductor d2[1,1;0]-( DMe- DCNQI) 2Cu

We have observed cyclotron resonance in the three dimensional organic conductor d 2[1,1;0]-( DMe- DCNQI) 2 Cu. An effective mass of (3.35±0.2) m e is obtained, in excellent agreement with the effective mass deduced from de Haas-van Alphen measurements. This result rules out the possibility that electron-electron interactions play an important role in these materials.

Cyclotron resonance in the organic metal α-(BEDT-TTF) 2TlHg(SCN) 4

Cyclotron resonance studies of the density-wave state of the organic metal α-(BEDT-TTF) 2TlHg(SCN) 4 have been carried out in the frequency range 50–100 GHz. Two strong resonances were observed, corresponding to cyclotron masses 1.35 m e and 0.5 m e. Quantum oscillations were also observed in the electrodynamic response at high magnetic fields; their frequency is very similar to that of the Shubnikov-de Haas oscillations seen in low frequency transport studies.

A tandem mass spectrometry and Fourier transform ion cyclotron resonance (FTICR) study of RMn+ (crown-ether) complexes in the gas phase

Two ion–molecule reactions were identified as the source of complexes of crown ethers with RMn+ (R = H, Br, OH, C6H5, acac, C5H5, C5H4CH3) ions in the gas phase. Adduct formation was the important route to stable RMn+(3n–crown–n) (n = 4,5,6) ions. Ligand substitution at the Mn atom took place when it was sterically accessible to the incoming macrocyclic molecule. The efficiency of the macrocyclic ligand exchange was also greatly affected by the coordinative vacancies occupied by the group R. The 5-coordinative C5H5-ligand completely prevented metal atom attack by a second macrocyclic molecule. Very slow ligand substitution was observed for (acac)2Mn+(3n–crown–n) and (acac)Mn+(3n–crown–n) ions (n = 4,5). Whenever ligand substitution occurred, it always involved the replacement of a smaller crown ether by a larger homologue. The unimolecular dissociation of RMn+(3n–crown–n) ions was studied by tandem mass spectrometry; it was dominated by the loss of neutral molecules (C2H4O, RH, RH + C2H2). These fragmenta...

Optically detected cyclotron resonance studies of multisubband quantum wells

We report optically detected cyclotron resonance (ODCR) measurements on two subband occupied electronic systems confined in triangular and square quantum wells. It is shown that the ODCR measurement is a useful tool for the understanding of the luminescence processes, in addition to the characterization of carrier concentration, effective mass and Fermi energy. We find that only one broad photoluminescence (PL) line locates below the energy gap for the triangular well, which is attributed to donor-to-acceptor recombinations. For the square well, two lines with emission energies higher than the bandgap are observed. Each line width is consistent with the Fermi energy of the first and second subband. This suggests that the emissions involve electron transitions from the first and second subbands at all occupied k states to localized holes. We also find that the effective mass of electrons in the first subband is heavier than that of electrons in the second subband in the triangular well due to the effect of nonparabolicity and the electron distribution, while the reverse is observed in the square well. This is explained in part by the nonparabolicity and, significantly, by the barrier leakage of the electron wavefunction. Furthermore, we find that the single particle relaxation time obtained from magnetoresistance measurements and the scattering time from cyclotron resonance are longer for electrons in the upper subband for both triangular and square wells, and the relaxation time is smaller than the corresponding scattering time, consistent with previous transport results.