On-resonance Excitation Research Articles

Theory of ion cyclotron resonance mass spectrometry: resonant excitation and radial ejection in orthorhombic and cylindrical ion traps

Most prior treatments of ion cyclotron resonance (ICR) radiofrequency excitation have been based on a uniform electric field (infinitely extended electrode) approximation. In this paper, we develop analytical representations (based on Taylor series or Fourier series expansions) for the radiofrequency excitation potential in finite orthorhombic traps of square cross-section. The rate of increase of ICR orbital radius in response to single-frequency on-resonance excitation was then integrated for these and other prior models to obtain the onset of radial ejection. Both present methods (and prior related methods by Rempel [Int. J. Mass Spectrom. Ion Processes, 70 (1986) 163] and van der Hart and van de Guchte [Int. J. Mass Spectrom. Ion Processes, 82 (1988) 17]) are nearly indistinguishable for ion cyclotron orbital radius out to about 40% of the maximum radius allowed by the trap, and diverge thereafter. The Fourier series method is highly accurate all the way out to the trap boundary for ions in the midplane of the trap; because the transverse component of the excitation field decreases away from that midplane, the Fourier series method represents an upper limit to the actual excited ICR orbital radius, and should thus accurately predict the onset of radial ejection. We find that the experimentally observed onset of ion ejection occurs at about 1.3 times the excitation voltage—duration product calculated from the uniform-field model, in excellent agreement with the Fourier series prediction. Moreover, since the experimentally observed onset of ejection is found to be essentially independent of trapping voltage and ion mass-to-charge ratio, m/z, the ejection onset can be interpreted as radial (rather than axial) ejection. The present theories are developed in full algebraic detail for a cubic ion trap. Extension of either method to orthorhombic traps of arbitrary relative dimensions is straightforward; explicit solutions are given for the Taylor series model for orthorhombic traps of square cross-section. In addition, the Fourier series method is extendable to right circular cylindrical traps of arbitrary aspect ratio; all relevant analytical expressions are listed in the text. Finally, for a given excitation voltage × duration product, the present results imply that the excitation period should exceed several ICR orbital periods in order to minimize magnetron radius; this constraint is especially important for ions of high m/z.

Rydberg series of autoionizing states in strong laser fields

We study the behavior of Rydberg series of resonances excited by intense laser fields. Our approach is based on a semiclassical formalism for the evolution in time of the atomic states coupled by the laser. The atomic system is described by a two-channel model in a Multichannel Quantum Defect Theory approach. We calculate the total ionization and the photoelectron spectrum after a certain interaction time. We present results both for on-resonance and off-resonance excitation of the series. We employ a more or less realistic pulse shape that corresponds to a narrow Fourier bandwidth. We show that the effects of the non-resonant members of the series on the photoelectron spectra can be important and we study it both as a function of laser intensity and as a function of the interaction time. We also show that our model correctly describes the Rabi oscillations between the ground state and the excitedAI state when the field is sufficiently strong.

Correlations in light emitted by three-level atoms.

Correlations between the emission of photons from an excited three-level atom with one transition more strongly driven than the other are studied. Antibunching, anticorrelation, and superbunching are exhibited in the four possible configurations: ``vee'' (V), ``lambda'' (\ensuremath{\Lambda}), and two cascades with the upper or lower transition strongly driven. We pay particular attention to the projection of the atom into particular states by the measurement of fluorescence, and its significance for detecting quantum jumps in the weak transition by observing the fluorescence from the strong transition. For incoherent excitation we find that cross antibunching (the anticorrelation between emissions of different types of photons) manifests itself as prolonged periods of darkness. For the V and the cascade configuration with a strongly driven lower transition, these periods herald the emission of a weak-transition photon; for the other cascade and the \ensuremath{\Lambda} configuration the periods are heralded by the emission of a weak-transition photon. For on-resonance coherent excitation, dark periods are extremely infrequent for the former two configurations, and are extremely long or infinite for the latter two.

NMR intensity measurements of half-integer quadrupole nuclei

The intensity of the NMR spectrum of a half-integer spin nucleus, subject only to the quadrupolar interaction, is calculated as a function of the length τ of the on-resonance excitation pulse and the quadrupole coupling paramters. For τ ⩽ π/ 4ω 1 ( I + 1/2) the intensity is proportional to τ and independent of the quadrupole coupling constant, whilst for larger τ the intensity passes through a maximum at a value which is dependent upon the parameters describing the quadrupole interaction. This is numerically calculated for I = 512. Values of the quadrupole coupling constant can be determined from measurements of the intensity as a function of τ.

Polarisation dependence in three-level atom resonance fluorescence

The resonance fluorescence from a three-level atom irradiated by a monochromatic laser is studied. Detailed spectra are calculated by using the Heisenberg source-field approach, and introducing Liouvillian space techniques. It is predicted that in the steady-state limit the fluorescence from an atom having two closely spaced excited states will have an anisotropically polarised seven-line spectrum. Graphs are shown of the predicted spectra in the case of on-resonance excitation and of far off-resonance excitation.

Some features of coherent optical effects in large molecules

In this paper we present a theoretical study of coherent optical effects such as optical free induction decay (OFID) and photon echoes (PE) from an ensemble of collision-free large molecules. We have introduced a generalized effective hamiltonian to account for the temporal characteristics of the macroscopic polarization. Explicit results have been obtained for the OFID and PE under short-time on-resonance optical pulsed excitation elucidating the nature of phase destruction effects originating from intramolecular interstate and intrastate coupling and from inhomogeneous broadening effects.