We present the first threshold ion-pair production spectrum of a triatomic molecule. We have recorded the ion-pair yield spectrum and TIPP spectrum for the H 2 S f H + +SHion-pair channel using single-photon excitation. From the TIPP spectrum, we have determined the H-SH bond energy (31451 ( 4 cm -1 ) to unprecedented accuracy and demonstrated the formation of weakly bound H + -SH -(J′) ion-pair states, with rotational excitation of the SHanion up to J′ ) 4. The bound nature of these states, and the assigned spectrum that results from their field dissociation suggests that this technique can be applied to many other triatomic and larger polyatomic molecules in the future, leading to energetic, spectroscopic, and dynamical information about these species.

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New measurements of the zero-energy electron yield in the region of the photodouble ionization threshold in helium are presented, showing for the first time a Wannier-type cusp with a small asymmetry of the excitation/ionization wing amplitudes, similar to the electron impact results in the region of the first ionization threshold. The observed pressure dependence of the cusp amplitude asymmetry and other second-order effects are discussed in an attempt to explain the discrepancy with the earlier photoionization results of Hall et al. (1991).

The (1+1) resonance enhanced multiphoton ionization spectra of jet-cooled CS2 were obtained between 45 500 and 48 000 cm−1 (220–208 nm) for the two isotopomers C3212S2 and C3212S32S. With the use of different expansion gases which resulted in varying degrees of vibrational cooling and the comparison of the two isotopomer spectra, a partial assignment of the C̃ 1B2–X̃ 1Σg+ transition was obtained. The electronic origin of this transition is confirmed to lie at 46 248.7 cm−1 and values for the predissociation lifetimes for the upper state for 34 vibrational bands and isotopomer shifts of seven vibrational levels of the B21 state are presented. The lifetimes of the Σ0 bands were found to be larger than those of corresponding Π1 and Δ2 bands. A simulation of the spectrum, which used the harmonic approximation, is in qualitative agreement with the band positions and shifts, but quantitative disagreement between the values leads us to conclude that a normal mode analysis is not appropriate to describe the low vibrational levels of the B21 state. The results presented increase our understanding of the low-lying predissociating levels of CS2, which will be of use in future photofragment studies.

A novel phenomenon is observed in the dynamics of laser-prepared coherent wave packets, bound by the Coulombic 1=r potential of an ion-pair system. After exciting weakly bound (3 meV) H F ÿ wave packets in a Stark field, and permitting them to evolve in time, control of field dissociation via adiabatic and diabatic routes is demonstrated by applying delayed pulsed-electric fields, involving a zero-field crossing. Control manifests itself through the production of ions from each pathway at a different instant in time. This phenomenon is applied to map the oscillatory behavior of an angular momentum wave packet in a heavy Rydberg system. The characteristic frequencies of the observed Stark oscillations verify predicted mass-scaling laws for heavy Rydberg systems.

Constructed-response (CR) questions are a mainstay of introductory physics textbooks and exams. However, because of the time, cost, and scoring reliability constraints associated with this format, CR questions are being increasingly replaced by multiple-choice (MC) questions in formal exams. The integrated testlet (IT) is a recently developed question structure designed to provide a proxy of the pedagogical advantages of CR questions while procedurally functioning as set of MC questions. ITs utilize an answer-until-correct response format that provides immediate confirmatory or corrective feedback, and they thus allow not only for the granting of partial credit in cases of initially incorrect reasoning, but, furthermore, the ability to build cumulative question structures. Here, we report on a study that directly compares the functionality of ITs and CR questions in introductory physics exams. To do this, CR questions were converted to concept-equivalent ITs, and both sets of questions were deployed in midterm and final exams. We find that both question types provide adequate discrimination between stronger and weaker students, with CR questions discriminating slightly better than the ITs. There is some indication that any difference in discriminatory power may result from the baseline score for guessing that is inherent in MC testing. Meanwhile, an analysis of interrater scoring of the CR questions raises serious concerns about the reliability of the granting of partial credit when this traditional assessment technique is used in a realistic (but nonoptimized) setting. Furthermore, we show evidence that partial credit is granted in a valid manner in the ITs. Thus, together with consideration of the vastly reduced costs of administering IT-based examinations compared to CR-based examinations, our findings indicate that ITs are viable replacements for CR questions in formal examinations where it is desirable both to assess concept integration and to reward partial knowledge, while efficiently scoring examinations.

We report the first study of electromagnetically induced transparency (EIT) on the D1 and D2 lines of 7 Li in a vapour cell. The effect of different polarizations, background gas pressures and experimental designs on the dark resonance are examined. It is found that EIT is more prominent on the D1 line than on the D2 line and is present at the D2 transition under incident orthogonal linearly polarized fields and parallel circularly polarized fields, but absent for parallel linearly polarized fields and for orthogonal circularly polarized fields. It is shown that the contrast of the dark resonances can be affected by the presence of an inert background gas. An analysis of a further nine resonances observed at the D1 transition and three resonances at the D2 transition is presented.