Biomedical subjects

H Helm

Publications and source records attributed to H Helm.

At least 19 recordsLinked to original sources

PubMed2005

Quality of parent-child relationship and adolescent HIV risk behaviour in St. Maarten.

The highest incidence rates of HIV/AIDS in the Caribbean are recorded in heterosexual and youth populations. With sparse prevention and intervention programmes in place, there is a pressing need to address the HIV/AIDS risk of youth. The objective of this analysis was to describe the extent of youth risk behaviour in St. Maarten and explore the relationship between quality of parental-child relationship and adolescent HIV risk behaviours. The sample consisted of 1,078 students (age range 14-18, mean 15.6 (s.d. 1.7). The data were collected by self-report survey in the Spring of 2001 in the classrooms of all seven secondary schools in St. Maarten. The survey instrument included demographic information, and used questions derived from the Center for Disease Control's (CDC) Youth Risk Behavior Survey (YRBS) to assess health risk behaviour prevalence, including tobacco, alcohol, and drug use, and sexual activity. The survey also asked youth to rate their relationship with their parents. Analysis showed a relatively high rate of risk behaviour in this school population. Multi-variate analysis showed that a 'great' relationship with both parents, as perceived by the student, was significantly associated with lower rates of tobacco and alcohol use as well as lower rates of sexual activity.

Adolescent↗

PubMed2004

Nonadiabatic coupling and vector correlation in dissociation of triatomic hydrogen.

We determine experimentally the vector correlation among the three neutral ground state hydrogen atoms which appear in dissociation of neutral H3* molecules. The sum of the kinetic energies of the three H-atoms is fixed by selecting the energy of the H3* molecule by laser excitation in the range between 0.85 and 3.60 eV. The highly structured maps of correlation in the motion of the three atoms provide a direct view of the internal molecular couplings which initiate dissociation. We discuss this feature in a model calculation and in terms of a new quantum chemical calculations of the potential energy surfaces of H3*.

Journal Article↗

PubMed2002

Collective vibrational modes in biological molecules investigated by terahertz time-domain spectroscopy.

We present well-resolved absorption spectra of biological molecules in the far-IR (FIR) spectral region recorded by terahertz time-domain spectroscopy (THz-TDS). As an illustrative example we discuss the absorption spectra of benzoic acid, its monosubstitutes salicylic acid (2-hydroxy-benzoic acid), 3- and 4-hydroxybenzoic acid, and aspirin (acetylsalicylic acid) in the spectral region between 18 and 150 cm(-1). The spectra exhibit distinct features originating from low-frequency vibrational modes caused by intra- or intermolecular collective motion and lattice modes. Due to the collective origin of the observed modes the absorption spectra are highly sensitive to the overall structure and configuration of the molecules, as well as their environment. The THz-TDS procedure can provide a direct fingerprint of the molecular structure or conformational state of a compound.

Anti-Inflammatory Agents, Non-Steroidal↗

PubMed2001

Photodetachment of H- in a strong infrared laser field.

Negative hydrogen ions are exposed to a short infrared laser pulse of 2.15 microm wavelength and 250 fs duration. An imaging technique is used to record the energy resolved angular distribution of photoelectrons. The energy spectrum reveals at least three excess photon detachment channels. A quantum interference effect, resulting in an unusual dependence of the angular distribution on the electron kinetic energy, is most prominent in the lowest two-photon detachment channel. The appearance of this effect is discussed in terms of the threshold law of photodetachment.

Journal Article↗

PubMed2000

Terahertz pulse propagation in the near field and the far field.

We present a detailed investigation of the propagation properties of beams of ultrashort terahertz (THz) pulses emitted from large-aperture (LA) antennas. The large area of the emitter is demonstrated to have substantial influence on the temporal pulse profile in both the near field and the far field. We perform a numerical analysis based on scalar and vectorial broadband diffraction theory and are able to distinguish between near-field and far-field contributions to the total THz signal. We find that the THz beam from a LA antenna propagates like a Gaussian beam and that the temporal profile of the THz pulse, measured in the near field, contains information about the temporal and spatial field distribution on the emitter surface, which is intrinsically connected to the carrier dynamics of the antenna substrate. As a result of pulse reshaping, focusing of the THz beam leads to a reduced relative pulse momentum, with implications in THz field-ionization experiments.

Computer Simulation↗

PubMed1999