H. Schmutz

Selective field ionization of high Rydberg states: Application to zero-kinetic-energy photoelectron spectroscopy

Sequences of pulsed electric fields have been designed and tested that enable a higher selectivity in the pulsed field ionization of high Rydberg states (n⩾100) than has so far been possible. The enhanced selectivity originates from the permutation of the parabolic quantum numbers n1 and n2 that is induced by a sufficiently rapid inversion of the electric field polarity during a pulse sequence. A reliable procedure, based on numerical simulations of the outcome of pulse field ionization sequences, has been developed to detect and control changes in the parabolic quantum numbers that can occur during a pulse sequence. The procedure can be used to assess under which conditions a clean permutation of the parabolic quantum numbers can be achieved. Unwanted randomization of m, n1 and n2, which reduces the selectivity of the field ionization process, can be avoided by minimizing the time intervals during which the electric field in the pulse sequence is almost zero. The high selectivity reached in the pulsed fi...

Very high resolution spectroscopy of high Rydberg states of the argon atom

Very high resolution spectra of high Rydberg states of the argon atom with principal quantum numbers in the range n=60–200 have been measured in double-resonance experiments using a high resolution vacuum ultraviolet laser and frequency stabilized millimeter waves. The 250 kHz resolution achieved in the double-resonance spectra enables the determination of accurate effective quantum numbers and the precise measurement of fine-structure intervals in l=0–3 Rydberg states at n values much beyond 50. The high resolution is also used to detect spectral shifts induced by small electric fields. Analysis of these spectral shifts allows the determination of stray electric fields with uncertainties of less than 1 mV/cm and their compensation to less than 1 mV/cm. The spectra of high Rydberg states are very strongly influenced by experimental conditions and the highest resolution can only be obtained when the stray electric fields are reduced to less than 1 mV/cm and the intensity of the millimeter waves are reduced...

Velocity-tunable slow beams of cold O2 in a single spin-rovibronic state with full angular-momentum orientation by multistage Zeeman deceleration

Cold samples of oxygen molecules in supersonic beams have been decelerated from initial velocities of 390 and 450 m s−1 to final velocities in the range between 150 and 280 m s−1 using a 90-stage Zeeman decelerator. (2 + 1) resonance-enhanced-multiphoton-ionization (REMPI) spectra of the 3sσ g 3Π g (C) two-photon transition of O2 have been recorded to characterize the state selectivity of the deceleration process. The decelerated molecular sample was found to consist exclusively of molecules in the J ′′ = 2 spin–rotational component of the X ground state of O2. Measurements of the REMPI spectra using linearly polarized laser radiation with polarization vector parallel to the decelerator axis, and thus to the magnetic-field vector of the deceleration solenoids, further showed that only the magnetic sublevel of the N′′ = 1, J ′′ = 2 spin–rotational level is populated in the decelerated sample, which therefore is characterized by a fully oriented total-angular-momentum vector. By maintaining a weak quantization magnetic field beyond the decelerator, the polarization of the sample could be maintained over the 5 cm distance separating the last deceleration solenoid and the detection region.

Multistage Zeeman deceleration of metastable neon

A supersonic beam of metastable neon atoms has been decelerated by exploiting the interaction between the magnetic moment of the atoms and time-dependent inhomogeneous magnetic fields in a multistage Zeeman decelerator. Using 91 deceleration solenoids, the atoms were decelerated from an initial velocity of 580 m/s to final velocities as low as 105 m/s, corresponding to a removal of more than 95% of their initial kinetic energy. The phase-space distribution of the cold, decelerated atoms was characterized by time-of-flight and imaging measurements, from which a temperature of 10 mK was obtained in the moving frame of the decelerated sample. In combination with particle-trajectory simulations, these measurements allowed the phase-space acceptance of the decelerator to be quantified. The degree of isotope separation that can be achieved by multistage Zeeman deceleration was also studied by performing experiments with pulse sequences generated for (20)Ne and (22)Ne.

Precision Spectroscopy in Cold Molecules: The Lowest Rotational Interval of He_{2}^{+} and Metastable He_{2}.

Multistage Zeeman deceleration was used to generate a slow, dense beam of translationally cold He_{2} molecules in the metastable a ^{3}Σ_{u}^{+} state. Precision measurements of the Rydberg spectrum of these molecules at high values of the principal quantum number n have been carried out. The spin-rotational state selectivity of the Zeeman-deceleration process was exploited to reduce the spectral congestion, minimize residual Doppler shifts, resolve the Rydberg series around n=200 and assign their fine structure. The ionization energy of metastable He_{2} and the lowest rotational interval of the X^{+} ^{2}Σ_{u}^{+} (ν^{+}=0) ground state of ^{4}He_{2}^{+} have been determined with unprecedented precision and accuracy by Rydberg-series extrapolation. Comparison with ab initio predictions of the rotational energy level structure of ^{4}He_{2}^{+} [W.-C. Tung, M. Pavanello, and L. Adamowicz, J. Chem. Phys. 136, 104309 (2012)] enabled us to quantify the magnitude of relativistic and quantum-electrodynamics contributions to the fundamental rotational interval of He_{2}^{+}.

Slow and velocity-tunable beams of metastable He 2 by multistage Zeeman deceleration

He2 molecules in the metastable a Σu state have been generated by striking a discharge in a supersonic expansion of helium gas from a pulsed valve. When operating the pulsed valve at room temperature, 77 K, and 10 K, the mean velocity of the supersonic beam was measured to be 1900 m/s, 980 m/s, and 530 m/s, with longitudinal velocity distributions corresponding to temperatures of 4 K, 1.9 K, and 1.8 K, respectively. The characterization of the population distribution among the different rotational levels of the a Σu state by high-resolution photoelectron and photoionization spectroscopy indicated a rotational temperature of about 150 K for the beam formed by expansion from the room-temperature valve and a bimodal distribution for the beam produced with the valve held at 10 K, with rotational levels up to N ′′ = 21 being populated. A 55-stage Zeeman decelerator operated in a phase-stable manner in the longitudinal and transverse dimensions was used to further reduce the beam velocity and tune it in the range between 100 and 150 m/s. The internal-state distribution of the decelerated sample was determined by recording and analyzing the photoionization spectrum in the region of the lowest ionization threshold, where it is dominated by resonances corresponding to autoionizing np Rydberg states belonging to series converging to the different rotational levels of the X Σu ground state of He + 2 . The deceleration process did not reveal any rotational state selectivity, but eliminated molecules in spin-rotational sublevels with J ′′ = N ′′ from the beam, J ′′ and N ′′ being the total and the rotational angular momentum quantum number, respectively. The lack of rotational state selectivity is attributed to the fact that the Paschen-Back regime of the Zeeman effect in the rotational levels of the a Σu state of He2 is already reached at fields of only 0.1 T.

Trapping deuterium atoms

Cold deuterium atoms in a supersonic beam have been decelerated from an initial velocity of 475 m/s to zero velocity in the laboratory frame using a 24-stage Zeeman decelerator. The atoms have been loaded in a magnetic quadrupole trap at a temperature of {approx}100 mK and an initial density of {approx}10{sup 6} cm{sup -3}. Efficient deceleration was achieved by pulsing the magnetic fields in the decelerator solenoids using irregular sequences of phase angles. Trap loading was optimized by monitoring and suppressing the observed reflection of the atoms by the field gradient of the back solenoid of the trap.

Chirped-pulse millimetre-wave spectrometer for the 140–180 GHz region

ABSTRACT A chirped-pulse millimetre-wave spectrometer working in the 140–180 GHz range has been developed for spectroscopic investigations of molecules and clusters in pulsed supersonic expansions. Its sensitivity is illustrated by a measurement of the relative intensities of pure rotational transitions in O,14N15N16O,15N14N16O, O and O in a natural NO sample and of pure rotational transitions of the carbon monoxide dimer (12C16O). GRAPHICAL ABSTRACT

Stark and Zeeman deceleration of neutral atoms and molecules

Argon and hydrogen atoms excited to Rydberg Stark states in supersonic expansions have been decelerated using inhomogeneous electric fields. In the case of hydrogen, the atoms have been decelerated from an initial velocity of 700 m/s to zero velocity in the lab frame using time-dependent inhomogeneous electric fields and subsequently stored in two- and three-dimensional traps. The dynamics of the Rydberg atoms in the traps and the phase-space characteristics of the decelerated atoms have been characterized by pulsed field ionization and imaging techniques. Multi-stage Zeeman deceleration of ground state H and D atoms has been demonstrated. Using this technique H atoms, traveling at 420 m/s, have been decelerated to half of their initial kinetic energy.