Researchers at Vrije Universiteit in Amsterdam, the Netherlands, have demonstrated precision quantum interference metrology using a train of pulses of the fourth harmonic of a Ti:sapphire femtosecond laser. The work, which they reported in the Jan. 21 issue of Science, opens the door to spectroscopy of atoms and ions in the extreme-UV and soft x-ray spectral regions and to the development of highly accurate atomic optical clocks operating at vacuum- and extreme-UV frequencies.To date, precision spectroscopy relies on the counting of optical cycles from an ultrastable CW laser with respect to an absolute frequency standard. The technique works well at infrared and visible wavelengths, but efforts to do the same at UV and shorter wavelengths have been hampered by the broad bandwidth of the short laser pulses that are required to generate these wavelengths using nonlinear processes such as high-harmonic generation.The university scientists overcame this obstacle by employing an excitation scheme based on pulses from a Ti:sapphire mode-locked frequency comb laser, rather than from a CW laser. Using an electro-optic modulator, they selected up to three pulses of 850-nm radiation from the mode-locked oscillator, which they amplified in a six-pass nonsaturating Ti:sapphire amplifier and converted to a wavelength of 212.55 nm using two nonlinear BBO crystals. The UV pulses excited a two-photon transition in a collimated 0.3-mm-wide krypton atomic beam, and a 100-ps pulse of 532-nm light from an Nd:YAG ionized the excited atoms, accelerating the ions into a time-of-flight mass spectrometer for counting.The difference in the phase evolution between the atomic excitation, which is dependent on the transition frequency, and the UV pulses that induced the excitation gave rise to an oscillating ion signal when the pulses were shifted in time or phase. Combining three sets of measurements performed at different repetition rates of the Ti:sapphire laser yielded a measurement of the absolute transition frequency in krypton with an uncertainty that was an order of magnitude smaller than previously reported using single nanosecond-long laser pulses.