The infrared (IR) spectrum of the isolated protonated neurotransmitter dopamine was recorded in the fingerprint range (570-1880 cm À1) by means of IR multiple photon dissociation (IRMPD) spectroscopy. The spectrum was obtained in a Fourier transform ion cyclotron resonance mass spectrometer equipped with an electrospray ionization source, which was coupled to a free electron laser (FEL). The spectroscopic studies are complemented by quantum chemical calculations at the B3LYP and MP2 levels of theory using the cc-pVDZ basis set. Several low-energy isomers with protonation occurring at the amino group are predicted in the energy range 0-50 kJ mol À1. Good agreement between the measured IRMPD spectrum and the calculated linear absorption spectra is observed for the two gauche conformers lowest in energy (DE) and free energy (DG) at both levels of theory, denoted gÀ1 and g+1. Minor contributions of higher lying gauche isomers cannot be ruled out spectroscopically but their calculated energies suggest only minor population in the sampled ion cloud. In all these gauche structures, one of the three protons of the ammonium group is pointing toward the catechol subunit, thereby maximizing the intramolecular NH-p interaction of the positive charge with the aromatic ring. In total, 16 distinct vibrational bands are observed in the IRMPD spectrum and assigned to individual normal modes of the energetically most stable gÀ1 conformer, with deviations of less than 24 cm À1 (average 11 cm À1) between measured and calculated frequencies. Comparison with neutral dopamine reveals the effects of protonation on the geometric and electronic structure.

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Since transition-metal impurity levels are found to be aligned with respect to each other within a group of isovalent semiconductor compounds, as if a common reference level existed for them, we propose to use this fictitious level for the band alignment in semiconductor heterojunctions. This rule leads to a valence-band discontinuity in Gai_ x Al x As/GaAs heterojunction of A£ v =(0.34 ±0.05)xAis g (AlAs/GaAs) in agreement with the most recent measurements. Predictions of several III-V on III-V and II-VI on II-VI heterojunction band-edge discontinuities are also given.PACS numbers: 73.40.Lq, 71.25.Tn, 71.55.Fr The band-edge discontinuity in semiconductor heterojunctions (HJ) is among the most important parameters characterizing such a structure. Both the experimental results and the predictions by different theoretical models scatter dramatically in spite of the great technological importance of these structures and the necessity of knowing a reasonably accurate value of the discontinuity for the purpose of device engineering. Recent reviews of the subject strongly reflect this situation. l~3 There is a clear indication that the band discontinuities are not only the result of the bulk properties of both constituents of the HJ, but may also depend on the way that the HJ is prepared. Among the most obvious of these variable parameters is the crystallographic orientation of the substrate, the possible reconstruction of the interface, a more or less graded transition from one constituent to the other, the formation of a dipole layer, and other technological factors as discussed by Bauer and Sang. 2 Therefore, two extreme approaches to the problem can be found.(i) The existing universal approaches 4 " 7 are only of limited value in predicting band-edge discontinuities.The key argument is that they are based only on the bulk properties of the constituents. None of them nor the other more elaborate theoretical computations 8 takes into account the complete situation present in reality at the interface (see, e.g., the review by Bauer and Sang 2 ).(ii) In certain well-selected and well-prepared HJ's the discontinuity can be related to bulk properties alone. These restrictions would include, for instance, the formation of an abrupt junction, the absence of extra dipole layers and, therefore, electrically neutral surfaces [e.g., (110) in tetrahedrally coordinated compounds] , and to some extent the choice of the constituents of the HJ from only one ' 'class" of semiconductors, e.g., III-V or II-VI compounds. In this case the band-edge discontinuities are expected to fulfill the properties of "linearity and transitivity" as discussed by Kroemer. 1 To separate the technology-dependent factors from the factors governed only by the bulk properties, it is necessary either to determine experimentally or to predict in a reasonably accurate way the latter contribution.From the experimental side, the achievements of molecular-beam-epitaxy or metal-organic chemicalvapor-deposition growth techniques allow for preparation of a...

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The validity of a recent proposal that transition-metal impurity levels in semiconductors may serve as a reference in band alignment in semiconductor heterojunctions is positively verified by using the most recent data on band offsets in the following lattice-matched heterojunctions: Ga& "Al"As/GaAs, In& "Ga"As~P& "/InP, In& "Ga"P/GaAs, and Cd& "Hg"Te/CdTe. The alignment procedure is justified theoretically by showing that transition-metal energy levels are effectively pinned to the average dangling-bond energy level, which serves as the reference level for the heterojunction band alignment. Experimental and theoretical arguments showing that an increasingly popular notion on transition-metal energy-level pinning to the vacuum level is unjustified and must be abandoned in favor of the internal referen-ce rule proposed recently [J. M. Langer and H. Heinrich, Phys. Rev. Lett. 55, 1414Lett. 55, (1985]are presented. overlayers on various semiconductors, they have obtained the relative valence-band offsets for various semiconductors, from which, in principle, any heterojunction band offset can be obtained. Even if the heterojunctions grown by them were of device quality, the measuring technique suffers from an inherent accuracy limit of about 0.2 eV.Their approach resembles, to some extent, the empirical procedure known as the electron-amenity rule. ' The key difference between the two approaches is that, in the latter approach, ' ' the reference is the zero vacuum lev-38 7723 1988 The American Physical Society 7724 LANGER, DELERUE, LANNOO, AND HEINRICH 38 el (the valence-band offset is given by the difference in the

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Infrared spectra of the isolated protonated flavin molecules lumichrome, lumiflavin, riboflavin (vitamin B2), and the biologically important cofactor flavin mononucleotide are measured in the fingerprint region (600-1850 cm(-1)) by means of IR multiple-photon dissociation (IRMPD) spectroscopy. Using density functional theory calculations, the geometries, relative energies, and linear IR absorption spectra of several low-energy isomers are calculated. Comparison of the calculated IR spectra with the measured IRMPD spectra reveals that the N10 substituent on the isoalloxazine ring influences the protonation site of the flavin. Lumichrome, with a hydrogen substituent, is only stable as the N1-protonated tautomer and protonates at N5 of the pyrazine ring. The presence of the ribityl unit in riboflavin leads to protonation at N1 of the pyrimidinedione moiety, and methyl substitution in lumiflavin stabilizes the tautomer that is protonated at O2. In contrast, flavin mononucleotide exists as both the O2- and N1-protonated tautomers. The frequencies and relative intensities of the two C=O stretch vibrations in protonated flavins serve as reliable indicators for their protonation site.