Physical Review A

Near-infrared multiphoton ionization of cesium

William R. Christian, R. N. Compton, John A. D. Stockdale, Joel C. Miller, C. Dewey Cooper, Xian Tang, P. Lambropoulos

Published Oct 1, 1984• 11 citations

Abstract

Experimental and theoretical results are presented for resonantly enhanced three-photon ionization of cesium atoms in the wavelength range from 830 to 950 nm. The accidental near resonance of the $6p^{2}P_{\frac{3}{2},\frac{1}{2}}$ levels at the one-photon energy and the near resonance at the two-photon level with the $6d^{2}D_{\frac{3}{2},\frac{5}{2}}$ states gives rise to large ionization signals and interesting anomalies in the relative intensities of the fine-structure levels. "Hybrid resonances" are observed which involve the production of $6p ^{2}P_{\frac{1}{2}}$ atoms from molecular cesium. One hybrid resonance involves ionization of the $6p ^{2}P_{\frac{1}{2}}$ state via an electric quadrupole transition ($6p ^{2}P_{\frac{1}{2}}\ensuremath{-}7p ^{2}P_{\frac{3}{2}}$). Both experimental and theoretical photoelectron angular distributions are compared for all of the observed resonances except for the $6p ^{2}P_{\frac{3}{2}}$ state where the signals were too weak for such a measurement. Agreement between experiment and theory is fair for three-photon ionization via the $6p^{2}P_{\frac{1}{2}}$ state. Comparison of experiment and theory for the case of the $6d^{2}D_{\frac{3}{2},\frac{5}{2}}$ states indicates that hyperfine coupling during the laser-pulse duration may be important. The angular distributions for the hybrid resonances show the general behavior predicted by theory but the features appear broadened.

DOI: 10.1103/physreva.30.1775

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Photoelectron angular distributions from multiphoton ionization of cesium atoms

R. N. Compton, John A. D. Stockdale, C. Dewey Cooper +2 more

Physical Review AOct 1, 198437 citations

Multiphoton ionization of cesium via quadrupole transitions: Photoelectron angular distributions and polarization effects on total ionization rates

A. Lyras, Bo-nian Dai, Xian Tang +5 more

Physical Review AJan 1, 19888 citations

Two-photon resonant three-photon ionization of the nd D2 states of cesium, rubidium, and sodium: Photoelectron angular distributions

Adila Dodhy, John A. D. Stockdale, R. N. Compton +3 more

Physical Review AApr 1, 19879 citations

Multiphoton ionization of rubidium

C. B. Collins, Stephen M. Curry, B. W. Johnson +5 more

Physical Review ANov 1, 197649 citations

Multiphoton ionization of cesium atoms above and below the two-photon ionization threshold

Adila Dodhy, John A. D. Stockdale, R. N. Compton

Two‐ and three‐photon ionization processes in cesium atoms using a single pulsed‐dye laser and a cesium atomic beam have been investigated. Photoelectron angular distributions have been measured for two‐photon resonant, three‐photon ionization via the nd states (n=12, 15, and 21), for two‐photon nonresonant transitions over the photoelectron energy range from ∼26−100 meV, and for ‘‘quasi‐free‐free’’ transitions.

AIP conference proceedingsJan 1, 19840 citations

Resonance shifts in the multiphoton ionization of cesium atoms

J. Morellec, Didier Normand, G. Petite

Physical Review AJul 1, 1976107 citations

Physical Review A

Near-infrared multiphoton ionization of cesium

William R. Christian, R. N. Compton, John A. D. Stockdale, Joel C. Miller, C. Dewey Cooper, Xian Tang, P. Lambropoulos

Published Oct 1, 1984• 11 citations

Publisher

Abstract

Experimental and theoretical results are presented for resonantly enhanced three-photon ionization of cesium atoms in the wavelength range from 830 to 950 nm. The accidental near resonance of the

6p^{2}P_{\frac{3}{2},\frac{1}{2}}

levels at the one-photon energy and the near resonance at the two-photon level with the

6d^{2}D_{\frac{3}{2},\frac{5}{2}}

states gives rise to large ionization signals and interesting anomalies in the relative intensities of the fine-structure levels. "Hybrid resonances" are observed which involve the production of

6p ^{2}P_{\frac{1}{2}}

atoms from molecular cesium. One hybrid resonance involves ionization of the

6p ^{2}P_{\frac{1}{2}}

state via an electric quadrupole transition (

6p ^{2}P_{\frac{1}{2}}\ensuremath{-}7p ^{2}P_{\frac{3}{2}}

). Both experimental and theoretical photoelectron angular distributions are compared for all of the observed resonances except for the

6p ^{2}P_{\frac{3}{2}}

state where the signals were too weak for such a measurement. Agreement between experiment and theory is fair for three-photon ionization via the

6p^{2}P_{\frac{1}{2}}

state. Comparison of experiment and theory for the case of the

6d^{2}D_{\frac{3}{2},\frac{5}{2}}

states indicates that hyperfine coupling during the laser-pulse duration may be important. The angular distributions for the hybrid resonances show the general behavior predicted by theory but the features appear broadened.

DOI: 10.1103/physreva.30.1775

Related papers

Photoelectron angular distributions from multiphoton ionization of cesium atoms

R. N. Compton, John A. D. Stockdale, C. Dewey Cooper +2 more

We present both experimental and theoretical studies of resonantly enhanced multiphoton ionization of cesium atoms. Photoelectron angular distributions for two-photon ionization via the one-photon-allowed $7p^{2}P_{\frac{1}{2},\frac{3}{2}}$ and $8p^{2}P_{\frac{1}{2},\frac{3}{2}}$ intermediate states and three-photon ionization via the two-photon-allowed $8d^{2}D_{\frac{5}{2},\frac{3}{2}}$ resonant intermediate states are reported. The photoelectrons are energy analyzed with a spherical-sector electrostatic energy analyzer with an energy resolution of ~ 0.1 eV and angular resolution of ~ \ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}. A straightforward calculation based upon a finestructure scheme gives excellent agreement with the measured angular distributions for the $^{2}P_{\frac{1}{2}}$ states. The laser pulse duration is comparable to the hyperfine precession period which allows the hyperfine coupling to partially destroy the anisotropy initially produced in the $^{2}P_{\frac{3}{2}}$ resonant intermediate states. Quantitative calculations including hyperfine coupling take this into account and provide an expression which gives a reasonable fit to the $^{2}P_{\frac{3}{2}}$ data.We present both experimental and theoretical studies of resonantly enhanced multiphoton ionization of cesium atoms. Photoelectron angular distributions for two-photon ionization via the one-photon-allowed $7p^{2}P_{\frac{1}{2},\frac{3}{2}}$ and $8p^{2}P_{\frac{1}{2},\frac{3}{2}}$ intermediate states and three-photon ionization via the two-photon-allowed $8d^{2}D_{\frac{5}{2},\frac{3}{2}}$ resonant intermediate states are reported. The photoelectrons are energy analyzed with a spherical-sector electrostatic energy analyzer with an energy resolution of ~ 0.1 eV and angular resolution of ~ \ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}. A straightforward calculation based upon a finestructure scheme gives excellent agreement with the measured angular distributions for the $^{2}P_{\frac{1}{2}}$ states. The laser pulse duration is comparable to the hyperfine precession period which allows the hyperfine coupling to partially destroy the anisotropy initially produced in the $^{2}P_{\frac{3}{2}}$ resonant intermediate states. Quantitative calculations including hyperfine coupling take this into account and provide an expression which gives a reasonable fit to the $^{2}P_{\frac{3}{2}}$ data.

Physical Review AOct 1, 198437 citations

Multiphoton ionization of cesium via quadrupole transitions: Photoelectron angular distributions and polarization effects on total ionization rates

A. Lyras, Bo-nian Dai, Xian Tang +5 more

We report measurements of photoelectron angular distributions via electric quadrupole transitions in cesium atoms using multiphoton excitation techniques. Experimental results are presented for the cases of one-photon resonant, two-photon ionization via the 6d ${\mathrm{}}^{2}$ ${D}_{3/2}$ and 6d ${\mathrm{}}^{2}$ ${D}_{5/2}$ electric quadrupole transitions and for one-photon resonant, three-photon ionization via the 5d ${\mathrm{}}^{2}$ ${\mathrm{D}}_{3/2}$ and 5d ${\mathrm{}}^{2}$ ${D}_{5/2}$ transitions. The ratios of the total ionization rates for linearly and circularly polarized light have also been measured for the 6d ${\mathrm{}}^{2}$ ${\mathrm{D}}_{3/2}$ and 6d ${\mathrm{}}^{2}$ ${D}_{5/2}$ states. A theoretical analysis and detailed comparison with the experimental results are given.We report measurements of photoelectron angular distributions via electric quadrupole transitions in cesium atoms using multiphoton excitation techniques. Experimental results are presented for the cases of one-photon resonant, two-photon ionization via the 6d ${\mathrm{}}^{2}$${D}_{3/2}$ and 6d ${\mathrm{}}^{2}$${D}_{5/2}$ electric quadrupole transitions and for one-photon resonant, three-photon ionization via the 5d ${\mathrm{}}^{2}$ ${\mathrm{D}}_{3/2}$ and 5d ${\mathrm{}}^{2}$${D}_{5/2}$ transitions. The ratios of the total ionization rates for linearly and circularly polarized light have also been measured for the 6d ${\mathrm{}}^{2}$${\mathrm{D}}_{3/2}$ and 6d ${\mathrm{}}^{2}$${D}_{5/2}$ states. A theoretical analysis and detailed comparison with the experimental results are given.

Physical Review AJan 1, 19888 citations

Two-photon resonant three-photon ionization of the nd D2 states of cesium, rubidium, and sodium: Photoelectron angular distributions

Adila Dodhy, John A. D. Stockdale, R. N. Compton +3 more

Energy-resolved photoelectron angular distributions have been measured for the nd ${}^{2}$ D states (n=11--23) in cesium and rubidium atoms for two-photon resonant, three-photon ionization under conditions where the spin-orbit fine structure was not resolved. The photoelectron angular distributions reveal strong fine-structure mixing effects when the time duration of the laser pulse is longer than the time corresponding to the inverse of the fine-structure splitting of the nd states involved. Three-photon ionization photoelectron angular distributions for sodium atoms have been studied both experimentally and theoretically for laser frequencies in which resonance enhancement occurs via the two-photon excited nd ${}^{2}$ D states (n=5--9). The laser pulse duration is \ensuremath{\sim}6 ns which is comparable to or less than the fine-structure mixing time for the unperturbed fine-structure nd ${}^{2}$ ${D}_{5/2}$ ,3/2 levels. In addition, the high laser powers \ensuremath{\sim} ${10}^{8}$ W/ ${\mathrm{cm}}^{2}$ employed result in alterations of the energy separations of the fine-structure levels (and cause corresponding changes in the mixing times) due to the ac Stark effect. A detailed theoretical analysis is presented and good agreement is obtained with the experimental results.Energy-resolved photoelectron angular distributions have been measured for the nd ${}^{2}$D states (n=11--23) in cesium and rubidium atoms for two-photon resonant, three-photon ionization under conditions where the spin-orbit fine structure was not resolved. The photoelectron angular distributions reveal strong fine-structure mixing effects when the time duration of the laser pulse is longer than the time corresponding to the inverse of the fine-structure splitting of the nd states involved. Three-photon ionization photoelectron angular distributions for sodium atoms have been studied both experimentally and theoretically for laser frequencies in which resonance enhancement occurs via the two-photon excited nd ${}^{2}$D states (n=5--9). The laser pulse duration is \ensuremath{\sim}6 ns which is comparable to or less than the fine-structure mixing time for the unperturbed fine-structure nd ${}^{2}$${D}_{5/2}$,3/2 levels. In addition, the high laser powers \ensuremath{\sim}${10}^{8}$ W/${\mathrm{cm}}^{2}$ employed result in alterations of the energy separations of the fine-structure levels (and cause corresponding changes in the mixing times) due to the ac Stark effect. A detailed theoretical analysis is presented and good agreement is obtained with the experimental results.

Physical Review AApr 1, 19879 citations

Multiphoton ionization of rubidium

C. B. Collins, Stephen M. Curry, B. W. Johnson +5 more

The multiphoton excitation of rubidium has been investigated over the 4600-6500-\AA{} wavelength region with a tunable dye-laser source having a linewidth better than 0.1 \AA{} and a space-charge ionization detector sensitive to a few ions per second. Multiphoton transitions have been observed to occur both through intermediate atomic states and through intermediate continuum states of the rubidium molecule. In the former case two-photon transitions have been observed from the $5^{2}S$ ground state of atomic rubidium to higher-lying $n^{2}D$ levels for values of $n$ ranging from 9 to 34 and to $n^{2}S$ levels for values of $n$ from 11 through 20. The fine-structure intervals of the $n^{2}D$ levels for $n=9 \mathrm{to}c 13$ were measured together with the line-strength ratios and were found to be in good agreement with the predictions of a simple theoretical model. At the shorter wavelengths hybrid two-photon resonances were observed to be excited through resonant intermediate continuum states of ${\mathrm{Rb}}_{2}$ . As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate $p\ensuremath{\pi}(^{3}\ensuremath{\Sigma}_{u1})$ and ${0}_{g}^{+}$ terms dissociating to give a $5{P}_{\frac{3}{2}}$ atom and resulting in the strong development of features corresponding to the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}D_{\frac{3}{2},\frac{5}{2}}$ part of the diffuse series, for $n\ensuremath{\le}50$ in absorption and the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}S_{\frac{1}{2}}$ part of the sharp series. Components to $32^{2}D$ and $30^{2}S$ were recorded to a precision of 0.3 ${\mathrm{cm}}^{\ensuremath{-}1}$ and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a $5^{2}P_{\frac{1}{2}}$ atom were not observed in the wavelength interval available in this experiment.The multiphoton excitation of rubidium has been investigated over the 4600-6500-\AA{} wavelength region with a tunable dye-laser source having a linewidth better than 0.1 \AA{} and a space-charge ionization detector sensitive to a few ions per second. Multiphoton transitions have been observed to occur both through intermediate atomic states and through intermediate continuum states of the rubidium molecule. In the former case two-photon transitions have been observed from the $5^{2}S$ ground state of atomic rubidium to higher-lying $n^{2}D$ levels for values of $n$ ranging from 9 to 34 and to $n^{2}S$ levels for values of $n$ from 11 through 20. The fine-structure intervals of the $n^{2}D$ levels for $n=9 \mathrm{to}c 13$ were measured together with the line-strength ratios and were found to be in good agreement with the predictions of a simple theoretical model. At the shorter wavelengths hybrid two-photon resonances were observed to be excited through resonant intermediate continuum states of ${\mathrm{Rb}}_{2}$. As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate $p\ensuremath{\pi}(^{3}\ensuremath{\Sigma}_{u1})$ and ${0}_{g}^{+}$ terms dissociating to give a $5{P}_{\frac{3}{2}}$ atom and resulting in the strong development of features corresponding to the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}D_{\frac{3}{2},\frac{5}{2}}$ part of the diffuse series, for $n\ensuremath{\le}50$ in absorption and the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}S_{\frac{1}{2}}$ part of the sharp series. Components to $32^{2}D$ and $30^{2}S$ were recorded to a precision of 0.3 ${\mathrm{cm}}^{\ensuremath{-}1}$ and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a $5^{2}P_{\frac{1}{2}}$ atom were not observed in the wavelength interval available in this experiment.

Physical Review ANov 1, 197649 citations

Multiphoton ionization of cesium atoms above and below the two-photon ionization threshold

Adila Dodhy, John A. D. Stockdale, R. N. Compton

AIP conference proceedingsJan 1, 19840 citations

Resonance shifts in the multiphoton ionization of cesium atoms

J. Morellec, Didier Normand, G. Petite

This work reports the multiphoton ionization of cesium atoms, via resonance with the $6F$ level, by interaction with a single transverse and longitudinal mode neodymium-glass laser beam. At a low rate of ion production, when the laser frequency is tuned through resonance, the laser intensity necessary to create a constant number of ions passes through a minimum, as expected, for a three-photon energy equal to the unperturbed $6S\ensuremath{\rightarrow}6F$ transition energy. But for a higher ionization rate, involving larger laser intensities, a departure from expectation is observed which is interpreted as a consequence of an energy shift of the transition. The level shifts are demonstrated in another way by their strong effect on the experimental order of nonlinearity of the interaction, ${K}_{\mathrm{expt}}$ . A sharp variation of ${K}_{\mathrm{expt}}$ is observed when the energy shift of the $6S\ensuremath{\rightarrow}6F$ transition exactly compensates the wavelength detuning of the laser. The latter method gives shift values which are found to be independent of the large variations of the interaction volume under conditions of resonance. Both methods give a linear dependence of the shifts on the laser intensity. Identification of these shifts with energy shifts of the $6S\ensuremath{\rightarrow}6F$ transition is subject to theoretical reservations and needs further experimentation.This work reports the multiphoton ionization of cesium atoms, via resonance with the $6F$ level, by interaction with a single transverse and longitudinal mode neodymium-glass laser beam. At a low rate of ion production, when the laser frequency is tuned through resonance, the laser intensity necessary to create a constant number of ions passes through a minimum, as expected, for a three-photon energy equal to the unperturbed $6S\ensuremath{\rightarrow}6F$ transition energy. But for a higher ionization rate, involving larger laser intensities, a departure from expectation is observed which is interpreted as a consequence of an energy shift of the transition. The level shifts are demonstrated in another way by their strong effect on the experimental order of nonlinearity of the interaction, ${K}_{\mathrm{expt}}$. A sharp variation of ${K}_{\mathrm{expt}}$ is observed when the energy shift of the $6S\ensuremath{\rightarrow}6F$ transition exactly compensates the wavelength detuning of the laser. The latter method gives shift values which are found to be independent of the large variations of the interaction volume under conditions of resonance. Both methods give a linear dependence of the shifts on the laser intensity. Identification of these shifts with energy shifts of the $6S\ensuremath{\rightarrow}6F$ transition is subject to theoretical reservations and needs further experimentation.

Physical Review AJul 1, 1976107 citations

Experimental and theoretical results are presented for resonantly enhanced three-photon ionization of cesium atoms in the wavelength range from 830 to 950 nm. The accidental near resonance of the

6p^{2}P_{\frac{3}{2},\frac{1}{2}}

levels at the one-photon energy and the near resonance at the two-photon level with the

6d^{2}D_{\frac{3}{2},\frac{5}{2}}

states gives rise to large ionization signals and interesting anomalies in the relative intensities of the fine-structure levels. "Hybrid resonances" are observed which involve the production of

6p ^{2}P_{\frac{1}{2}}

atoms from molecular cesium. One hybrid resonance involves ionization of the

6p ^{2}P_{\frac{1}{2}}

state via an electric quadrupole transition (

6p ^{2}P_{\frac{1}{2}}\ensuremath{-}7p ^{2}P_{\frac{3}{2}}

). Both experimental and theoretical photoelectron angular distributions are compared for all of the observed resonances except for the

6p ^{2}P_{\frac{3}{2}}

state where the signals were too weak for such a measurement. Agreement between experiment and theory is fair for three-photon ionization via the

6p^{2}P_{\frac{1}{2}}

state. Comparison of experiment and theory for the case of the

6d^{2}D_{\frac{3}{2},\frac{5}{2}}

7p^{2}P_{\frac{1}{2},\frac{3}{2}}

and

8p^{2}P_{\frac{1}{2},\frac{3}{2}}

intermediate states and three-photon ionization via the two-photon-allowed

8d^{2}D_{\frac{5}{2},\frac{3}{2}}

resonant intermediate states are reported. The photoelectrons are energy analyzed with a spherical-sector electrostatic energy analyzer with an energy resolution of ~ 0.1 eV and angular resolution of ~ \ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}. A straightforward calculation based upon a finestructure scheme gives excellent agreement with the measured angular distributions for the

^{2}P_{\frac{1}{2}}

states. The laser pulse duration is comparable to the hyperfine precession period which allows the hyperfine coupling to partially destroy the anisotropy initially produced in the

^{2}P_{\frac{3}{2}}

resonant intermediate states. Quantitative calculations including hyperfine coupling take this into account and provide an expression which gives a reasonable fit to the

^{2}P_{\frac{3}{2}}

data.

{\mathrm{}}^{2}

{D}_{3/2}

and 6d

{\mathrm{}}^{2}

{D}_{5/2}

electric quadrupole transitions and for one-photon resonant, three-photon ionization via the 5d

{\mathrm{}}^{2}

{\mathrm{D}}_{3/2}

and 5d

{\mathrm{}}^{2}

{D}_{5/2}

transitions. The ratios of the total ionization rates for linearly and circularly polarized light have also been measured for the 6d

{\mathrm{}}^{2}

{\mathrm{D}}_{3/2}

and 6d

{\mathrm{}}^{2}

{D}_{5/2}

states. A theoretical analysis and detailed comparison with the experimental results are given.

Energy-resolved photoelectron angular distributions have been measured for the nd

{}^{2}

D states (n=11--23) in cesium and rubidium atoms for two-photon resonant, three-photon ionization under conditions where the spin-orbit fine structure was not resolved. The photoelectron angular distributions reveal strong fine-structure mixing effects when the time duration of the laser pulse is longer than the time corresponding to the inverse of the fine-structure splitting of the nd states involved. Three-photon ionization photoelectron angular distributions for sodium atoms have been studied both experimentally and theoretically for laser frequencies in which resonance enhancement occurs via the two-photon excited nd

{}^{2}

D states (n=5--9). The laser pulse duration is \ensuremath{\sim}6 ns which is comparable to or less than the fine-structure mixing time for the unperturbed fine-structure nd

{}^{2}

{D}_{5/2}

,3/2 levels. In addition, the high laser powers \ensuremath{\sim}

{10}^{8}

{\mathrm{cm}}^{2}

employed result in alterations of the energy separations of the fine-structure levels (and cause corresponding changes in the mixing times) due to the ac Stark effect. A detailed theoretical analysis is presented and good agreement is obtained with the experimental results.

5^{2}S

ground state of atomic rubidium to higher-lying

n^{2}D

levels for values of

n

ranging from 9 to 34 and to

n^{2}S

levels for values of

n

from 11 through 20. The fine-structure intervals of the

n^{2}D

levels for

n=9 \mathrm{to}c 13

were measured together with the line-strength ratios and were found to be in good agreement with the predictions of a simple theoretical model. At the shorter wavelengths hybrid two-photon resonances were observed to be excited through resonant intermediate continuum states of

{\mathrm{Rb}}_{2}

. As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate $p\ensuremath{\pi}(^{3}\ensuremath{\Sigma}_{u1})$ and

{0}_{g}^{+}

terms dissociating to give a

5{P}_{\frac{3}{2}}

atom and resulting in the strong development of features corresponding to the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}D_{\frac{3}{2},\frac{5}{2}}$ part of the diffuse series, for $n\ensuremath{\le}50$ in absorption and the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}S_{\frac{1}{2}}$ part of the sharp series. Components to

32^{2}D

and

30^{2}S

were recorded to a precision of 0.3 ${\mathrm{cm}}^{\ensuremath{-}1}$ and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a

5^{2}P_{\frac{1}{2}}

atom were not observed in the wavelength interval available in this experiment.

This work reports the multiphoton ionization of cesium atoms, via resonance with the

6F

level, by interaction with a single transverse and longitudinal mode neodymium-glass laser beam. At a low rate of ion production, when the laser frequency is tuned through resonance, the laser intensity necessary to create a constant number of ions passes through a minimum, as expected, for a three-photon energy equal to the unperturbed $6S\ensuremath{\rightarrow}6F$ transition energy. But for a higher ionization rate, involving larger laser intensities, a departure from expectation is observed which is interpreted as a consequence of an energy shift of the transition. The level shifts are demonstrated in another way by their strong effect on the experimental order of nonlinearity of the interaction,

{K}_{\mathrm{expt}}

. A sharp variation of

{K}_{\mathrm{expt}}

is observed when the energy shift of the $6S\ensuremath{\rightarrow}6F$ transition exactly compensates the wavelength detuning of the laser. The latter method gives shift values which are found to be independent of the large variations of the interaction volume under conditions of resonance. Both methods give a linear dependence of the shifts on the laser intensity. Identification of these shifts with energy shifts of the $6S\ensuremath{\rightarrow}6F$ transition is subject to theoretical reservations and needs further experimentation.