Frontiers in Spectroscopy

Chemical Physics 880 and 880A

Winter 2008

Course Description: This course will provide students with an overview of topics on the frontier of spectroscopic research. It will exploit internationally renowned lecturers, as well as outstanding OSU faculty, to cover topics ranging from very fundamental to quite applied. General areas to be covered will include fundamental characteristics of molecular quantum structure, electromagnetics, new experimental techniques, remote sensing, ultra-high sensitivity analytical techniques, astrophysical applications, etc. It is planned that the course will be offered multiple times, with topics and speakers varying with each offering. The lecturers for the upcoming Winter quarter are listed below.

Each topic will be covered by lectures on Wednesday and Friday mornings, 9:00-10:18AM, in MP2015.

Thursdays discussions this year only will begin at 9:00-10:18AM on Thursdays in MP2015.

Prerequisites: a previous spectroscopy course at OSU in Chemistry or Physics or prior permission of the instructor

Required Text: None; suggested articles for reading will be supplied prior to the lecture on a given topic.

All readings and lecture notes will be kept current on Carmen. If you were enrolled for Frontiers this quarter, you have your normal student login and password. If you are an advisor or post-doc and would like access to the readings and notes, email Becky Gregory, gregory.10@osu.edu, and she will arrange for a login and password for this site.

List of speakers and dates scheduled:

January 16-18 Marsha Lester, University of Pennsylvania

• Lecture 1 - Infrared action spectroscopy, stability, and dissociation dynamics of the HOOO radical intermediate
  This laboratory has recently obtained the first infrared spectrum of the hydrogen trioxide (trans-HOOO) radical, an intermediate invoked in the H + O₃ and O + HO₂ atmospheric reactions as well as the efficient vibrational relaxation of OH radicals by O₂. There had been much debate in the literature as to whether HOOO is stable or metastable with respect to the OH + O₂ limit, as well as the relative stability of the cis and trans conformers. By measuring the OH product state distribution following IR excitation of HOOO, we have directly determined the stability of trans-HOOO and shown that is much greater than prior estimates. As a result, HOOO may act as temporary sink for OH radicals and be present in measurable concentrations in the Earth's atmosphere. The experimental stability indicates that up to 50% of the OH radicals in the vicinity of the tropopause may be bound to O₂, rather than free OH radicals. Our work has continued with studies of HOOO in the fundamental OH stretch region, including combination bands that reveal information on low frequency torsional and bending modes, and analogous studies of DOOO.

• Lecture 2 - Spectroscopic characterization of peroxynitrous acid and nitric acid by infrared overtone spectroscopy
  This laboratory has obtained high-resolution spectra of peroxynitrous acid (HOONO), a key intermediate in atmospheric chemistry. HOONO had been proposed as a secondary product of one of the most important reactions in the chemistry of the Earth's lower atmosphere, namely OH + NO2-> HONO₂, which controls the conversion of reactive NO₂ into chemically inactive nitric acid (HONO₂). A significant production of HOONO is expected to have a profound impact on modeling of NOx chemistry in the lower atmosphere, including its role in the ozone budgets of both the troposphere and stratosphere. Our spectroscopic studies of HOONO in the OH overtone region provide the first definitive identification of the trans-perp (tp) conformer of HOONO. In addition, the OH overtone excitation imparts sufficient energy to dissociate tp-HOONO, allowing us to determine the HOONO bond energy and thereby estimate its yield (up to 20%) under atmospheric conditions. Further studies have identified transitions associated the cis-perp (cp) conformer of HOONO, providing insight on the complex torsional motion and conformational dynamics of HOONO when compared with theoretical predictions. Parallel studies on HONO₂ with complementary theory have identified the three strongly coupled states involved in a Fermi resonance that initiate rapid intramolecular vibrational energy redistribution following OH overtone excitation.

• Lecture 3 - Electronic quenching of OH A ²Σ⁺ radicals in single collision events with molecular hydrogen

  Collisional quenching of electronically excited OH A²Σ⁺ radicals has been extensively investigated because of its impact on OH concentration measurements in atmospheric and combustion environments. Yet little is known about the outcome of these events, except that they facilitate the efficient removal of OH population from the excited A ²Σ⁺ electronic state by introducing nonradiative decay pathways. The quenching of OH A ²Σ⁺ by H₂ and D₂ has emerged as a benchmark system for studying the nonadiabatic processes that lead to quenching. Theoretical calculations indicate that a conical intersection funnels population from the excited to ground electronic surfaces. Previously, this laboratory observed bimodal Doppler profiles for the H/D-atom products of reactive quenching, which was attributed to direct and indirect dynamical pathways through the conical intersection region. Our recent work is focused on characterizing the nonreactive quenching process, where OH X²Π products are generated with a remarkably high degree of rotational excitation and lambda-doublet specificity. The OH quantum state distribution directly reflects the anistropy and A' symmetry of the conical intersection region. Surprisingly, we also find that reaction accounts for nearly 90% of the quenched products.

February 6-8 Peter Barker, University College London

• Lecture 1
  The manipulation of the centre-of-mass motion of molecules is becoming increasingly important for a range of new applications in physics, chemistry, and engineering. These include high-resolution spectroscopy, the creation of cold molecules, chemistry at ultra-cold temperatures, and for rapid measurements of thermodynamic parameters in high-speed flows. In this lecture I will provide a theoretical framework for understanding how intense (10¹⁰ -10¹² W/cm² ), non-resonant light fields can be used to manipulate essentially any polarizable particle. In particular I will discuss the central role of molecular polarizability and the production of molecular alignment in strong fields which can be used to modify the forces on molecules.

• Lecture 2
  In this lecture I will describe important applications, with emphasis on the creation of cold ensembles of molecules by deceleration of a molecular beam using pulsed optical fields. I will describe how these slowed molecules can be trapped and further cooled using a range of schemes including cooling by refrigeration with laser-cooled cold atoms and with optical cavities. Included in this lecture will be an overview of the emerging field of molecule optics in which optical elements for focusing and diffraction of molecules can be created by tailored light fields.

• Lecture 3
  In this lecture I will discuss new laser-based methods for gas-phase diagnostics based on the controlled manipulation of molecules in the gas phase using optical forces. I will focus on a technique called coherent Rayleigh scattering (CRS) which is used to infer thermodynamic properties such as gas temperature from the light scattered from the perturbations to the gas induced by optical forces. I will describe the spectral properties of the CRS signal and also the spectral effects of briefly trapping these molecules in intense optical fields, including Dicke narrowing and the creation of vibrational sidebands. Finally, I will discuss a new type of mass spectrometry based on polarizability-to-mass ratio.

February 13-15 Steve Boxer, Stanford University

• Link to Abstracts from Frontiers website

• Link to Abstracts from Carmen website

February 20-22 Jeremy Hutson, University of Durham

• Lecture 1 (Wed) - Cooling molecules
  Reading:
  1. Review on "Production and application of translationally cold molecules"
  2. Short paper on "Ultracold Rb-OH collisions and prospects for sympathetic cooling"
  Reasons for studying cold molecules. Stark and Zeeman effects: high-field-seeking and low-field-seeking states. Methods for cooling to millikelvin: Stark deceleration, buffer gas cooling, velocity selection (optical deceleration will already have been covered by Peter Barker). Traps for cold molecules. How to get to microkelvin? The problem of inelastic collisions and the need for collision studies.

• Lecture 2 (Fri)- Controlling ultracold collisions
  Reading: review on "Molecule formation in ultracold atomic gases"
  Cold atom studies. Formation of molecules in ultracold atomic gases: photoassociation and magnetoassociation. Scattering resonances and the scattering length. Bound states near dissociation. Avoided crossings. Manipulating bound states. Prospects for producing ultracold molecules in the ground rovibrational state. Polar molecules. Controlled ultracold chemistry.

February 27-29 Thomas Rizzo, Ecole Polytechnique Federale de Lausanne

• Lecture 1 (Wed) - Spectroscopic studies of cold, gas-phase biological ions and their hydrates
  After a brief introduction on the spectroscopy of biological molecules in the gas phase, this first lecture will introduce the techniques used to produce and cool closed-shell biological ions. The electronic spectroscopy and photophysics of individual amino acid ions and their hydrates with water will be discussed and implications for condensed phase systems will be drawn. The use of IR-UV double-resonance for conformational determination will also be introduced.

• Lecture 2 (Fri) - Conformation-specific infrared spectroscopy of gas-phase peptides.
  This lecture will focus on IR-UV double-resonance spectra of gas-phase peptides ranging from simple dimers up to helical 12-mers. The focus will be on the use of IR spectroscopy, together with theory, to determine the gas phase structure. Experiments in which laser-driven transfer of population between different stable conformations will also be discussed.

Grading: Satisfactory/Unsatisfactory options: Class attendance and participation

Letter grade option: Class attendance and participation plus term paper

(Grades will be assigned solely by OSU faculty.)  

(3 hours) Call number 04450-5 for ChemPhys 880 (S/U option)