Unexpected Challenges (2000)

Since the first attempts to calculate the helium ground state in the early days of Bohr-Sommerfeld quantization, two-electron atoms have posed a series of unexpected challenges to theoretical physics. Despite the seemingly simple problem of three charged particles with known interactions, it took more than half a century after quantum mechanics was established to describe the spectra of two-electron atoms satisfactorily. The evolution of the understanding of correlated two-electron dynamics and its importance for doubly excited resonance states is presented here, with an emphasis on the concepts introduced. The authors begin by reviewing the historical development and summarizing the progress in measuring the spectra of two-electron atoms and in calculating them by solving the corresponding Schrödinger equation numerically. They devote the second part of the review to approximate quantum methods, in particular adiabatic and group-theoretical approaches. These methods explain and predict the striking regularities of two-electron resonance spectra, including propensity rules for decay and dipole transitions of resonant states. This progress was made possible through the identification of approximate dynamical symmetries leading to corresponding collective quantum numbers for correlated electron-pair dynamics. The quantum numbers are very different from the independent particle classification, suitable for low-lying states in atomic systems. The third section of the review describes modern semiclassical concepts and their application to two-electron atoms. Simple interpretations of the approximate quantum numbers and propensity rules can be given in terms of a few key periodic orbits of the classical three-body problem. This includes the puzzling existence of Rydberg series for electron-pair motion. Qualitative and quantitative semiclassical estimates for doubly excited states are obtained for both regular and chaotic classical two-electron dynamics using modern semiclassical techniques. These techniques set the stage for a theoretical investigation of the regime of extreme excitation towards the three-body breakup threshold. Together with periodic orbit spectroscopy, they supply new tools for the analysis of complex experimental spectra.

Affording dental, hearing, and vision care is also an issue among adults 65 and older as those benefits are not generally covered by Medicare.1 See the October 2021 Health Tracking Poll for a deeper dive into health care costs and challenges among older adults.

Many U.S. adults may be one unexpected medical bill from falling into debt. About half of U.S. adults say they would not be able to pay an unexpected medical bill that came to $500 out of pocket. This includes one in five (19%) who would not be able to pay it at all, 5% who would borrow the money from a bank, payday lender, friends or family to cover the cost, and one in five (21%) who would incur credit card debt in order to pay the bill. Women, those with lower household incomes, Black and Hispanic adults are more likely than their counterparts to say they would be unable to afford this type of bill. (Source: KFF Health Care Debt Survey: Feb.-Mar. 2022)

The overall share of adults able to withstand small financial emergencies was similar to pre-pandemic levels; however, financial challenges remained for the millions of Americans who have been laid off in the past year. After rising in July above pre-pandemic levels while many relief policies were in place, the share of adults able to withstand small financial emergencies returned to pre-pandemic levels by November. However, many Americans faced a substantial financial setback when they lost their jobs during the pandemic. Those who were laid off then faced lower rates of preparedness for additional financial emergencies both large and small. Among laid-off workers, Black and Hispanic workers and those with less education, in particular, faced challenges covering additional financial emergencies.

Being unable to pay bills on time can have serious consequences. For renters facing challenges paying their bills, 36 percent (4 percent of all adults and 16 percent of all renters) said they would not have anywhere to go if they had to move out of their home because they could no longer make their rent payments. (See the "Housing" section of this report for more on evictions).

Those who were laid off in the 12 months before the survey were more likely to have difficulty meeting their regular monthly expenses. Forty-five percent of adults who were laid off in the prior year were unable to pay their bills in full or would have been unable to do so if faced with an unexpected $400 expense, versus 24 percent of those who were not laid off.

Differences in ability to pay bills among those who were laid off differed substantially across education levels. Among those who were laid off, more than 6 in 10 with a high school degree or less were unable to pay their bills in full or would have been unable to do so if faced with an unexpected $400 expense, compared with 24 percent of adults with at least a bachelor's degree.

Black and Hispanic adults who were laid off were also less well positioned to handle an additional financial setback compared to the overall population of laid-off workers (figure 19). Among laid-off Black workers, 64 percent were unable to pay their bills in full or would have been unable to do so if faced with an unexpected $400 expense, compared with 45 percent of all laid-off adults.

Some financial challenges, such as a job loss, require more financial resiliency than would an unexpected $400 expense. One common measure of financial resiliency is whether people have savings sufficient to cover three months of expenses if they lost their primary source of income. Fifty-five percent of people said they had set aside money specifically as emergency savings or "rainy day" funds. However, those who experienced a layoff may have dipped into those funds or not had them in the first place (see box 1). Forty percent of adults who were laid off in the past 12 months could not cover three months of expenses by any means were they to lose their job or government benefits. Among adults who were not laid off, a lower share (28 percent) could not cover these expenses.

Out-of-pocket spending for health care is a common unexpected expense that can be a substantial hardship for those without a financial cushion, particularly during a global pandemic threatening both families' health and financial stability. As with the financial setbacks discussed earlier, many adults were not financially prepared for health-related costs at the time of the survey.

Seventeen percent of adults had major, unexpected medical expenses in the prior 12 months, with the median amount between $1,000 and $1,999. Sixteen percent of adults had debt from their own medical care or that of a family member, potentially from more than a year ago. Moreover, millions of Americans had contracted COVID-19, and 6 percent of all adults paid out-of-pocket for COVID-19-related medical care.

Health insurance is one way that people can pay for routine medical expenses and protect against the financial burden of large, unexpected expenses. In 2020, 90 percent of adults had health insurance, largely unchanged from 2019. The majority of adults had health insurance through their employers. However, many adults lost jobs in 2020 and may have also lost health insurance. Nearly one in five adults who had been laid off in the past 12 months were uninsured as of November 2020.

Cloud transformation is business transformation, and getting it right is more important than ever. However, organizations of all sizes and industries face unique challenges when undertaking cloud transformation projects, as well as when managing and securing their more complex multicloud and hybrid architectures.

While the recovery of Synechococcus in the anoxic/dark conditions found in the deep Black Sea was novel and unexpected, it is consistent with the fact that different oxygen concentrations have led to the development of aerobic, micro-oxic and anerobic variants of the enzymes involved in the tetrapyrrole (chlorophylls, hemes and bilins) biosynthesis in Cyanobacteria [47]. Indeed, Synechococcus sp. PCC 7002, Leptolyngbya boryana, Cyanothece sp. PCC 7425, Anabaena variabilis, Synechocystis sp. PCC 6803 and other Cyanobacteria contain the vast majority of the aerobic, micro-oxic and anaerobic pathways for tetrapyrrole biosynthesis [47]. Our findings illuminate the metabolic pathways encoded in the genome of our strains, and shed light on their relation with the observed environmental conditions and Chl a signal.

In conclusion, considerable concordance between the four human papillomavirus assays was observed for triage indications in women with abnormal cytology. However, in primary screening of women above age 30 substantial differences in detecting human papillomavirus infections were observed for the same assays. Knowing that the use of primary human papillomavirus testing could provide a number of benefits for cervical screening, this finding is nonetheless an unexpected challenge that will need to be addressed.

Videoconferencing can help rural physicians gain access to quality continuing medical education and continuing professional development. It can also provide for peer-to-peer consultation and communication. There are unique advantages and challenges that presenters and facilitators using videoconferencing need to be aware of to maximize this interactive learning opportunity. There are also advantages and challenges to three current approaches to videoconference-based peer-to-peer consultation. Through telehealth initiatives that effectively use vidoeconferencing, rural physicians and patients alike can overcome geographic isolation and benefit from a considerable and well-deserved increase in access to CME and a wider spectrum of health services. 2b1af7f3a8