Double resonant absorption measurement of acetylene symmetric vibrational states probed with cavity ring down spectroscopy

A novel mid-infrared/near-infrared double resonant absorption setup for studying infrared-inactive vibrational states is presented. A strong vibrational transition in the mid-infrared region is excited using an idler beam from a singly resonant continuous-wave optical parametric oscillator, to populate an intermediate vibrational state. High output power of the optical parametric oscillator and the strength of the mid-infrared transition result in efficient population transfer to the intermediate state, which allows measuring secondary transitions from this state with a high signal-to-noise ratio. A secondary, near-infrared transition from the intermediate state is probed using cavity ring-down spectroscopy, which provides high sensitivity in this wavelength region. Due to the narrow linewidths of the excitation sources, the rovibrational lines of the secondary transition are measured with sub-Doppler resolution. The setup is used to access a previously unreported symmetric vibrational state of acetylene, nu(1) + nu(2) + nu(3) + nu(1)(4) + nu(-1)(5) in the normal mode notation. Single-photon transitions to this state from the vibrational ground state are forbidden. Ten lines of the newly measured state are observed and fitted with the linear least-squares method to extract the band parameters. The vibrational term value was measured to be at 9775.0018(45) cm(-1), the rotational parameter B was 1.162 222(37) cm(-1), and the quartic centrifugal distortion parameter D was 3.998(62) x 10(-6) cm(-1), where the numbers in the parenthesis are one-standard errors in the least significant digits. Published by AIP Publishing.Peer reviewe

Observation of the effect of gravity on the motion of antimatter

Einstein’s general theory of relativity from 19151 remains the most successful description of gravitation. From the 1919 solar eclipse2 to the observation of gravitational waves3, the theory has passed many crucial experimental tests. However, the evolving concepts of dark matter and dark energy illustrate that there is much to be learned about the gravitating content of the universe. Singularities in the general theory of relativity and the lack of a quantum theory of gravity suggest that our picture is incomplete. It is thus prudent to explore gravity in exotic physical systems. Antimatter was unknown to Einstein in 1915. Dirac’s theory4 appeared in 1928; the positron was observed5 in 1932. There has since been much speculation about gravity and antimatter. The theoretical consensus is that any laboratory mass must be attracted6 by the Earth, although some authors have considered the cosmological consequences if antimatter should be repelled by matter7,8,9,10. In the general theory of relativity, the weak equivalence principle (WEP) requires that all masses react identically to gravity, independent of their internal structure. Here we show that antihydrogen atoms, released from magnetic confinement in the ALPHA-g apparatus, behave in a way consistent with gravitational attraction to the Earth. Repulsive ‘antigravity’ is ruled out in this case. This experiment paves the way for precision studies of the magnitude of the gravitational acceleration between anti-atoms and the Earth to test the WEP

An extreme-ultraviolet frequency comb enabling frequency metrology with highly charged ions

Highly charged ions (HCI) have been proposed as extremely sensitive probes for physics beyond the Standard Model, such as a possible α-variation, and as novel frequency standards, due to their insensitivity to external fields. We aim at performing ultra-high precision spectroscopy of HCI in the extreme ultraviolet (XUV) region, where many transitions are located. Therefore, we have developed an XUV frequency comb. Femtosecond pulses from a 100 MHz phase-stabilized near-infrared comb are amplified and fed into an enhancement cavity inside an ultra-high vacuum chamber. In the tight focus (w0 = 15 μm) of the astigmatism-compensated cavity, intensities ~10^14 W/cm^2 are reached. As a first application, we perform multi-photon ionization of xenon using the velocity-map imaging technique. The high repetition rate facilitates fast data acquisition even at low intensities, enabling future precision tests in nonlinear physics. Finally, we have observed outcoupled XUV radiation, produced in the cavity focus, up to the 35th harmonic order (42 eV; 30 nm). No signs of mirror degradation were observed during five hours of continuous operation. Using He:Xe gas mixtures, improved phase-matching conditions led to 49 μW output power at 16 eV. This is sufifcient to drive HCI transitions with kHz excitation rates and is an important step towards XUV frequency metrology with HCI