The Forbidden Transition in Ytterbium

1

• The Forbidden Transition in Ytterbium
• Atomic selection rules forbid E1 transitions
  between states of the same parity. However, the
  parity-violating weak interaction between the
  nucleons and electrons can mix states of opposite
  parity, resulting in a small parity-violating E1
  transition amplitude.
• An external electric field also mixes states of
  opposite parity. This results in a
  Stark-induced transition amplitude. This much
  larger transition amplitude can be interfered
  with the small parity-violating transition
  amplitude allowing observation of the small
  parity-violating effects.
  
  
• Why ytterbium?
• In ytterbium, the odd parity state 6s6p 1P1 state
  is near in energy to the even parity 5d6s 3D1
  state (see energy diagram). In perturbation
  theory, the mixing of these states is enhanced by
  the small energy denominator.
• The high charge of the ytterbium nucleus (Z 70)
  is important since the parity-violating effects
  scale as Z3. The parity violating effect is
  expected to be 10 and 100 times larger than
  those previously studied in thallium and cesium,
  respectively.
• Ytterbium has seven stable isotopes (A168, 170,
  171, 172, 173, 174, 176) and the parity-violating
  effects are expected to be different for each
  isotope. This limits the dependence of the
  measurement upon atomic structure calculations,
  which are currently less precise than
  experimental measurements.

2

• Parity Nonconservation In Atoms
• Within an atom there is an interaction due to the
  weak force. This interaction occurs via the
  exchange of virtual Z-bosons between the
  electrons and nucleons within an atom. Because
  this interaction does not conserve parity the
  parity of atomic states, as defined by the
  electromagnetic interaction, is not completely
  preserved.
• The presence of a parity-violating interaction
  mixes states of opposite parity. This mixing is
  manifested in the optical properties of the atom.
  
• Because the Standard Model predicts the size of
  these parity-violating effects, precision
  measurements of atomic parity violation provide a
  low-energy test for the Standard Model and may be
  sensitive to physics beyond the Standard Model.

3

• Work In Progress
• Stark-induced E1 Amplitude
• In order to determine the parity-violating
  effects on an absolute scale we are currently
  working on measuring the Stark-induced transition
  amplitude. We use a c.w. laser to excite the 6s2
  1S0 ? 5d6s 3D1 transition (408nm) in an effusive
  atomic beam (see diagram) within the presence of
  an electric field and measure the absorption.
• To calibrate the density of the atomic beam we
  measure the absorption of 556nm light on the 6s2
  1S0 ? 6s6p 3P1 transition. This absorption
  coefficient is known from the lifetime of the
  6s6p 3P1 state.
• Given the branching ratios of the decay of the
  5d6s 3D1 state, we can also use fluorescence to
  measure the Stark-induced amplitude. The atoms
  in the excited 5d6s 3D1 state decay through the
  6s6p 3P2, 1, 0 states to the 6s2 1S0 ground
  state. Comparing the fluorescence from the 6s6p
  3P1? 6s2 1S0 transition (556nm), after exciting
  with 408nm light, with the fluorescence from the
  6s6p 3P1? 6s2 1S0 transition (556nm), after
  exciting with 556nm light, allows for a second
  method of measurement of the Stark-induced
  amplitude.
• M1 Transition Amplitude
• Determining the parity-violating amplitude by
  observing the interference with the Stark-induced
  amplitude requires a small M1 amplitude so that
  the parity-nonconserving amplitude is not masked
  by the M1 amplitude.
• The M1 amplitude for the 6s2 1S0 ? 5d6s 3D1
  transition is estimated to be highly suppressed,
  but a direct measurement of the M1 amplitude is
  necessary do determine any effect its presence
  may have on the parity nonconservation
  measurement.

4
Current Experimental Apparatus
5
Observation of the Forbidden Transition
This plot shows the fluorescence from the 6s6p
3P1 ? 6s2 1S0 transition after exciting the 3D1
state. The fluorescence is observed with a
photomultplier tube as the excitation-laser
frequency is scanned.
6
Low-Lying Energy Levels of Ytterbium
6s5d 3D3
6s6p 1P1
PNC and Stark Mixing
6s5d 3D2
6s5d 3D1
6s6p 3P2
6s6p 3P1
408 nm
6s6p 3P0
556 nm
6s2 1S0
Odd
Even
7
Investigation of the 6s2 1S0 ? 5d6s 3D1
Transition in Atomic Ytterbium C.J. Bowers, D.
Budker, E. D. Commins, D. DeMille, S.J. Freedman,
G. Gwinner, J.E. Stalnaker
8
Stark Shift Measurement
This plot shows the effect of the electric field
on both the amplitude and position of the
transition for the case of Yb171 1/2 ? 1/2. The
electric field is switched between 40kV and 25kV
throughout the scan. Points are connected to
show time sequence. Each point corresponds to a
2 second time period.
9

• Results
• Lifetime Measurements
• In order to determine the branching ratios the
  lifetimes of 21 excited states in atomic
  ytterbium were measured using time-resolved
  fluorescence detection after pulsed laser
  excitation (C.J. Bowers et.al. Phys. Rev. A 53,
  3103(1995)).
• Stark Shifts
• We have measured the Stark shifts of the 6s2 1S0
  ? 5d6s 3D1 transition (408nm). This is done by
  exciting with laser light at 408nm and observing
  the cascade fluorescence at 556nm while varying
  the electric field.
• In order to minimize the effects of temperature
  drifts of the laser frequency, we switch the
  electric field between two values as we scan over
  the resonance (see Stark shift plot).
• The size of the shifts are 20 MHz for the values
  of the electric field used (20-50kV).
• Isotope Shifts and Hyperfine Structure
• Our experimental setup allows us to measure the
  isotope shifts and hyperfine structure for the
  5d6s 3D1 states. This is done by exciting the
  6s2 1S0 ?5d6s 3D1 transition and observing the
  fluorescence of the 6s6p 3P1? 6s2 1S0 transition
  with the photomultiplier tube.

10
408 nm Transmission Photodiode Detector
556 nm Transmission Photodiode Detector
Chopping of the atomic beam allows lock-in
detection of both fluorescence and absorption
signals Normalization of laser power reduces
noise in absorption signals due to laser power
fluctuations. Low laser power avoids optical
pumping and saturation effects. Fluorescence
detection during a calibrated laser frequency
scan (with increased 408nm laser power) is used
for measurement of hyperfine structure, isotope
shifts, and Stark shifts.
Mirror
Dichroic Mirror
Atomic Beam Chopper Wheel
Holes in Field Plates to See Fluorescence
Fluorescence Detection PMT
Electric Field Plates (45kV/cm)
Atomic Beam
556 nm
Atomic Oven
E
408 nm Normalization Photodiode Detector
408nm Laser Beam (20mW)
Beamsplitter
Dichroic Mirror
556 nm Normalization Photodiode Detector
556nm Laser Beam (2nW)
Mirror
Beamsplitter