Ralph C. Shiell

AN INFRARED SPECTRUM OF THE MOLECULAR DICATION (DOUBLY POSITIVELY CHARGED MOLECULE), D35CL2+

We have observed an infrared spectrum within the X 3Σ− state of DCl2+ using a fast-ion-beam/laser-beam spectrometer. A preliminary analysis shows good agreement with the rotational constants and tunneling lifetimes calculated by Bennett and McNab for the ν=1–2 vibrational band, although the calculated band origin appear to be in error by 21.1 cm−1.

High resolution spectroscopy and structure of molecular dications

The structure and reactivity of molecular dications has been the subject of increasing attention from theoreticians and experimentalists. We consider vibrationally resolved spectra of molecular dications, with particular emphasis on the interpretation of the observed spectral lines, before reviewing in detail all known high resolution spectra of molecular dications. We give a progress report on the state of our own calculations and ion beam measurements of the molecular dications DCl2+ and NO2+.

Microcontroller-based wavemeter using compression locking of an internal mirror reference laser

We show that a highly reliable and compact wavemeter incorporating a reference laser and traveling Michelson interferometer can be controlled, and the unknown wavelength determined, using readily programmable microcontroller circuits. These chips, powered by a 5V supply, each replace a network of many electronic components and enable equipment to be built and debugged in a very short time, to occupy a very small footprint and to be easily modified. With this wavemeter we demonstrate the ease with which programmable microcontrollers can play an important role in a physics research laboratory. We also present, for the first time, the stabilization of an internal mirror HeNe reference laser through direct compression of the glass tube, resulting in stable, mode power balanced operation for a period of more than 8h. This novel stabilization scheme enables a much higher bandwidth and shorter settling time of the servo system than previous heater-based approaches by overcoming technical problems associated with...

Pulsed field ionization-photoion spectroscopy using two-bunch synchrotron radiation: Time-of-flight selection scheme

We have demonstrated that the time-of-flight (TOF) selection method for pulsed field ionization (PFI) photoelectron detection [Jarvis et al., Rev. Sci. Instrum. 70, 2615 (1999)] can also be applied for the detection of PFI-photoions (PFI-PIs) using the two-bunch synchrotron radiation at the Advanced Light Source. By employing the supersonic beam technique to lower the translational temperature of the sample gas, we show that background prompt ions formed in direct and spontaneous autoionization processes arrive at the ion detector in a pattern similar to that of the vacuum ultraviolet light bunches. The PFI-PIs formed at dark gaps can be designed to arrive at the detector in between adjacent prompt ion peaks, enabling the gating of the PFI-PI signal with only minor contamination from background prompt ions. This experiment has revealed important considerations for the design of a general TOF selection scheme for PFI-PI detection using synchrotron radiation.

Optical pumping and electromagnetically induced transparency in a lithium vapour

We report the first study of electromagnetically induced transparency (EIT) on the D1 and D2 lines of 7Li 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.

Coaxial ion beam/infrared laser beam spectrometer for investigating infrared spectra of doubly positively charged molecules (molecular dications)

We have constructed an apparatus for studying the infrared spectra of molecules with a doubly positive charge (molecular dications). The spectroscopic transitions were recorded indirectly by means of observing a change in the fragmentation rate of the molecular dication when a transition was in resonance. The design and performance of the spectrometer are described, with particular emphasis on the sensitivity achieved for detecting infrared spectra and Zeeman split infrared spectra. The operation and calibration of the spectrometer are discussed and sample results for DCl2+ are presented. It is shown that we achieve the maximum possible signal/noise ratio that could be achieved in this type of experiment.

Resonance enhanced multiphoton ionization spectroscopy of jet cooled 12C32S2 and 12C34S32S from 45 500 to 48 000 cm−1

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 12C32S2 and 12C34S32S. 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 C 1B2–X 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 1B2 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 v...

Compact angle-resolved optical spectrum analyzer using a programmable microcontroller approach

We describe the characterization of a highly sensitive and adaptable angle-resolved optical spectrum analyzer constructed using off-the-shelf components. The flexible nature of programmable microcontrollers is exploited by this device, resulting in a diagnostic tool that can determine the mode structure from a variety of lasers, and monitor laser beam profiles in real time. A readily available CMOS image sensor provides sensitive detection of a Fabry?Perot interference pattern, and outputs a digital signal suitable for processing by these microcontrollers. In place of an oscilloscope, a considerably less expensive component-level LCD is used which is addressed by a microcontroller. Using a solid etalon with a free spectral range of 210?GHz, we demonstrate the ability of the device to determine the mode structure of an external cavity diode laser in real time. We show this device to be competitive with a commercial optical spectrum analyzer, in terms of both dynamic range and sensitivity. The low incident laser power required by this system (0.75??W) can typically be obtained from a back reflection from an optic, and this compact, low-cost device allows for the provision of a dedicated spectrum analyzer for each laser system of interest.

The double ionization cusp in the threshold photoelectron spectrum of helium

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).

Comparison of Integrated Testlet and Constructed-Response Question Formats.

Constructed-response (CR) questions are a mainstay of introductory physics textbooks and exams. However, because of 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. Meanwhile, an analysis of inter-rater 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 non optimized) 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 to both assess concept integration and to reward partial knowledge, while efficiently scoring examinations.